Prior to slitting one's wrists? Bryter Later, Who knows where the time etc, even. Good songs, but a wee bit maudlin, no wonder you need Terry D to, hopefully, liven you up.
And, wot, no Lovin' Spoonful: both "Daydream" and "Summer in the City" are shoo-ins for any summer playlist.
(I don't know the Serge Trousers song you mention, but if it is about black butterflies, I somewhat fear it is likely to all end in tears......)

languorous

...'Summer's Cauldron' and 'Grass' hit the spot for me. After that wonderful Yardbirds/Zombies performance I saw last night (both bands were brilliant, though I could have done without The Zombies' guitarist Keith Airey's occasional showboating...) I'm going to nominate 'Time Of The Season' too.

Do you need tunes if your lying on the grass, surely the wind in the trees, the song of birds, the sound of younger family members playing happily is more than enough!
If however you are talking about that time on a warm evening with the curtains gently blowing, you've all caught a bit of the sun and come back from somewhere nice you've a glass in hand what would I listen to then well, a version of Summer Samba might be good, or some Epic 45 or some st etienne or some bach perhaps.

and as for songs for a summer morning before going to the beach....

Don't you realise that Mungo Jerry must be in the list?
You know the song I mean.
Unlike drinking and driving, it's mandatory, by law.

Oh, I've been thinking about my life,
What's been wrong and what's been right,
Some say that and some say this,
Some say no and some say yes,
Alright, alright, alright,
Alright, alright, alright,

I don't know what's going on,
But I know what's right from wrong,
Too many head's and too many minds,
Too many wrongs and too many rights,
Alright, alright, alright,
Alright, alright, alright

I hear you talking, alright, alright, alright,
I hear you talking, alright, alright, alright

Alright, alright, alright, alright, alright,
Alright, alright, alright,
Alright, alright, alright, alright, alright,
Alright, alright, alright,

Oh, I've been thinking about my life,
What's been wrong and what's been right,
Some say that and some say this,
Some say no and some say yes,
Alright, alright, alright,
Alright, alright, alright,

I hear you talking, alright, alright, alright,
I hear you talking, alright, alright, alright
Alright, alright, alright, alright, alright,
Alright, alright, alright,
Alright, alright, alright, alright, alright,

(Love it!)

But I prefer Lady Rose

Baby Jump.
(God I hate a smartarse, placing the extremely well fitting cap on well-chiselled head)

Jollification by The Lightning Seeds.

by The Commodores always makes a sunny day brighter still. I was walking past the Students Union the other day just as the guitar break came in - you know the one: doobadadoooo....wipp. That one.

And in a similar vein, the Isleys' "Summer Breeze" always manages to deliver exactly what it says on the tin.

I'd also go along with most of the above, especially Mungo Jerry and XTC's "Skylarking" album.

I'll chuck on my old reggae or Motown or a bit of Aretha. But my absolute favourite summer sound is probably the first Saint Etienne album. I had that on heavy rotation the summer of 91 and it always makes me think of heat.

Orbital's Insides album does the trick for me too, especially those slightly cooler summer mornings or late evenings.

is the sun shining somewhere then?
I suppose Summer Set by Acker Bilk's a non-starter?

Big Log and Style Councils Long Hot summer take me back to a wonderful holiday in Cornwall and are somehow perfect.Touch of Grey by Grateful Dead was the song most played on the radio when I artrived in Sunny Miami for a 2 year work contract about 20 years ago - still reminds of sunshine despite the title.

is spot on for lying on a beach somewhere. I tested it fully at its time of release in San Sebastian and its a fully fledged summer classic.

Slightly random but I like 'em. More of an urban summer, rather than rural tranquility action. Hot sticky scenes, you know what I mean?

California Soul - Marlena Shaw
I Walk The Earth - King Biscuit Time
Happy House - Juan Maclean
New Grass - Talk Talk
Repent - Pedro
Someone Great - LCD Soundsystem
Together In Electric Dreams - Phil Oakey & Giorgio Moroder
You Got The Love - Source & Candi Staton
Blind - Hercules & Love Affair
Talking With Myself - Electribe 101
Thieves Like Us - New Order
Pull Up To The Bumper - Grace Jones
Northern Lites - Super Furry Animals
Girls Girls Girls - Jay Z
Roscoe (Wizards Sleeve mix) - Midlake
Genius Of Love - Tom Tom Club
Time To Pretend - MGMT
Yes It's You - Sweet Charles
Take Me With U - Prince & The Revolution
Who Loves The Sun - Velvet Underground

Life Is Sweet - Chemical Brothers
Sing It Back - Moloko
Endless Endless - Kraftwerk
Think Twice - Ralph Myerz
Just My Imagination - Temptations
Deep Blue Day - Brian Eno
Morgengruss - Popol Vuh
Dry The Rain - Beta Band
House Or Cards - Radiohead
Also Sprach Zarathrusta - Deodato
Kosmos (Brendan Lynch mix) - Paul Weller
Boy With The Arab Strap - Belle & Sebastian
Les Fleur - Minnie Riperton
Can You Hear Me - David Bowie
You'll Never Know - Hi Gloss
Good Life - Inner City
Roast Fish & Cornbread - Lee Scratch Perry
Heard It Through the Grapevine - The Slits
Black & Gold - Sam Sparro
Ascension (Don't Ever Wonder) - Maxwell

cos Together In Electric Dreams always reminds me of autumn.

Of course, anything tropicalia or bossa nova is an obvious shoo-in too.

pure-lightning seeds
summer in siam-the pogues
shorley wall-ooberman
the joker-steve miller band
its all so frivolous-xtc
cool in the pool-holger czuckey
bel air-can
man in the hills-burning spear
wonderful land-the shadows
cool runnings-bunny wailer
perfect-lightning seeds
starry eyes-the records
que pasa(me no popeye,you no olive oil)-coati mundi
out of my mind-buffalo springfield
lying on a beach-joel plaskett
on the beach-kirsty mccoll
the ghetto walk-miles davis
12.30 young girls are coming to the canyon-scott mckenzie
sunny goodge street-donovan
groovin-aretha franklin
country air-the beach boys
ventura highway-america
summer fling-kd lang
mango in the garden-bruce cockburn
everything reminds me of my dog- jane siberry
world in motion-new order
getting away with it-electronic
something in the air thunderclap newman
tunnel into summer-kimberley rew
365 is my number now-king sunny ade
float on-the floaters
anyone for tennis-cream
dont let the sun catch you crying-jose feliciano

and for a full monty side one "Sketches of Spain"

This Dylan album suits summer like no other.

...I bought the latest Paul Simon anthology (40 tracks, 10 quid, no real clinkers) yesterday and played it as I taxied my family around a sunny Cardiff. And it was perfect - I'd forgotten how bright and shiny a lot of his stuff is, even the more reflective songs, while Take Me To The Mardi Gras, Late In The Evening & That was Your Mother are right up there with the greatest summery tunes. Strange that he's got this reputation for being a bit glum.
While I'm on, Peel used to play Beach Baby by First Class (really) a lot in the summer 1974 and Rose Of Cimarron by Poco in '76 - both of which I can't hear without being transported back in time to younger, hairier & much, much lazier times.

Lying In The Sun - Stereophonics and Summer by Kelly Jones
Summer - War
Boys Of Summer - Don Henley
Girls In Their Summer Clothes - Springsteen (Magic!)
Sitting In The Park - Georgie Fame
Albatros - Fleetwood Mac
Summer Skin - Death Cab For Cutie
Summer's First Breath - Epic 45 (courtesy of a Word free CD)
Staring At The Sun – Rooster
and of course, the unsurpassable Here Comes The Sun (the George/Giles Martin remix is a great version).

...would have to be 'Summertime' by the Fresh Prince. Of course, when I found it heavily sampled 'Summer Madness' by Kool and The Gang, it has been surpassed.

Rich

... of mine is 'Summertime' by The Honeyz. Similar groove to Jazzy Jeff. Of course, the quintessential summer sound is 'The Girl From Ipanema'by Astrud Gilberto.

my favourites from the last few years (often both jolly and introspective) are

James Yorkston and the Athletes - Moving Up Country (not his bset album, but his best summer album)
Rilo Kiley - The Execution of all Things
Malcolm Middleton - Into the Woods
Monkey Swallows the Universe - The Bright Carvings (oh and the Casket Letters)

And this summer's indiegirlpopsuperspectacular is Slow Down Tallhassee - The Beautiful Light

more in the guilty pleasures vein, EMF's Schubert Dip is one of my all time summer faves.

'This Old Heart Of Mine (Is Weak For You)' by the Isley Bros is probably the definitive summer track for me - it doesn't have any specific summer connotations, but it's such a bouncy, feelgood track that evokes the spirit of a warm summer's day like nothing else.

The First Picture Of You does it for me every time. The whole album is rather wonderful.