This is a tough one, and you should go back and look at a lot of other earlier posts on here about this very question, it may give you some ideas. Its hard to do it, but I always say 1 day at U is better than 0!!!! But my advice is to make sure you stay on site that night so you can use your room key for fast passes, to get to the front of the lines to all the rides. Remember too U is 2 parks not one. And just get up early and hustle. I presonally think Islands of Adventure is the best park, but there are all sorts of opinions on here.
I guess the must do rides in my opinion are: Spider man, Jurassic park Boat ride, the Mummy, the Hulk, Men In Black, Shrek and Terminator shows, and many others but thats a good start.