So, if I had to spend 24 hours in an online game, hm...I'll go with Kingdom of Loathing. Even though it has cussing (which I don't like) whereas Neopets doesn't, at least you don't have to buy virtual money with real money (not since the last time I was on, anyway).
But if I could spend 24 hours in any video game, here are some I might pick:
Super Mario World - I've played through that game so many times that it'll be interesting to see it from the inside. Also, I've always wanted to ride a Yoshi.
New Super Mario Bros. Wii - See above re: Yoshi.
Super Smash Bros. Brawl - I'd get to personally beat up Lucario for replacing Mewtwo, and I've always wondered what my Final Smash would be. Being metal sounds cool too.
Jumping on Mushrooms - Being able to jump infinitely high but only fall a certain distance looks fun for some reason.
Super Mario Galaxy - Rosalina is HOT.