I have been there the last two years at that exact time ( my wife is a teacher and its here spring break)
The area can vary in temperature but, it has usually been in the 50's with some cold nights. I would suggest that you book your camping spots ahead of time as last year we got stuck day hiking cause it was all booked when we got there. You can call the rangers, they are great. If you dont' get to the back country for camping they have sites there or you can stay at the Out Post which is right before you enter the park.
My favorite hike is a nice long hike starting at the Squaw Flats going towards EC1, and make my way to Chesler Park staying at CP5 if you can get it. That site is awesome!! its an old cowboy camp and you get to see the "Negro bill" wanted sign.
After that I would go do the Joint trail which is cool. Then come back and gone down to Lost Canyon where you can usually find water that time of year. SQ2 is a cool site as is LC1 farther down. Good exposed slick rock and beautiful scenery.
It is such a cool place you just can't go wrong. If you need any book ideas or anything else let me know, the Needles are my stomping grounds.