Oh gods, so many ideas, so little time.
1) A water pistol, the type you have to pump for about six hours before you actually get to, you know, play with the thing.
I'd like to be able to shoot jets of water out of my hands.
I'd be able to swim like a fish, too. A fish that can shoot jets of water out of it's fins.
2) Fire itself. I'd be able to turn into flame, like the Human Torch, only without the flying, because I still don't quite understand how his toasty superpowers enable flight...
Downside to this, of course, is that I'd probably accidentally set fire to my own bedding. A lot.
3) While I've always wanted Spiderman's webslinging abilities, I feel that having eight legs might put a bit of a cramp in my social life, so I think, instead of being spliced with an actual spider, I'd want to be spliced with whatever bit of the spider makes the webbing.
Downside to this: Spider silk REALLY can't support 130 pounds of costume-clad teen jumping from a rooftop in a determined sort of way.