The happy, peppy songs of... Pink Floyd

Indeed. “Julia Dream”, by Roger, is one of their darkest and “classicest”-sounding songs from the Singles-Saucerful of Secrets era. It’s about drugs, sure, but the ominousness of the trip portrays nothing of Syd’s whimsy and optimism.

Do they have to be happy and peppy and bursting with love?

You’re thinking of Pink Felix.

My favorite PF song is Summer '68 (off of Atom Heart Mother), and though it’s more than a little wistful, it’s forward momentum consists of some upbeat, absolutely lovely major chord progressions.

Though Roger didn’t write that either, IIRC.

How about “Several Species of Small Furry Animals Gathered Together in a Cave and Grooving with a Pict”? I don’t think I’ve ever listened to that song without giggling.

But then I don’t remember ever listening to it without being stoned, either, so…

I always interpreted San Tropez as being sung by a corpse riding in a hearse.

If “by Roger” includes his solo work, Sunset Strip is reasonably upbeat and optimistic, in a nostalgic sort of way.

And, alas, has never been performed live. I am quite fond of it as well, even though its message is how much PF loathed their fans.

I disagree, but then I think we are talking about happy, peppy music, not lyrics.
Both these songs might have inspirational, positive lyrics but are not happy peppy-sounding songs (at least not to my ears).
In contrast Arnold Layne and See Emily Play have pretty disturbing or even depressing lyrics but are very upbeat happy little tunes.