Now that you're 21, theres a lot more to do. lol. First off, you're finally tall enough to get on all the rides, and there are alot to see, especially since in May, Mount Everest will finally be open. But, Im sure you already know about all the rides. As for when the sun goes down...
There is Downtown Disney, a collection of bars/clubs with an array of different sounds for you to enjoy. Also theres a movie theater, and theres DisneyQuest which is a bunch of innovative new video games and such with a disney theme.
I dont know how long youre going for, but if you havent been since you were two, there is a lot of ground to cover,lol.
And, May is a great month to do it.I'll also be down there at the end of May and look forward to the sun (Its snowing like crazy here, now...lol.)
Hopefully that helps!
If it doesnt...let me know. lol