It's Gonna Get Better is another safe bet, possibly after Land Of Confusion. I'm guessing there will be as many happy tunes and as few sad ones as possible. If The Knife was rehearsed they may have gotten to, "Some of you are going to d....wait, let's not do this one. Too soon."
Squonk, Watcher, and most songs from The Lamb are possible with help from the backup singers and audience.
I like thewatcher's idea of Phil doing a Willy Wonka walk-on. At the very least I'd like to see him do a short walk for I Can't Dance.
I can see why you might think It's Gonna Get Better might be a 'safe bet' because of its sentiments. However as a song, is it something that fans desperately want the band to play? I would have thought not. Then you mention 'happy' songs and say 'most songs from the Lamb'. Do you really think the Lamb songs are 'happy'?