It depends on what you are looking for and where you are staying. If staying near Heavenly, Kirkwood would be a good option at that time of year. Kirkwwod has quite a few saklow lifts, but some good terrain for all abilities. Closer to Squaw is Alpine Meadows which has a lot of good spring skiing (though it could still be winter then - you just never know). It is similar to Squaw with a lot of bowls. It would be best to hook up with people who know Squaw or Alpine and get them to show you around or rent an instructor. Squaw also has free mountain tours. At that time of year, they are not likely to be to busy.
Another option would good advanced to expert terrain is Sugar Bowl. Northstar is an option if you want any or all of the following: long groomers; more of a resort experience (nice village); varied Apres ski activirties (like ice skating, music, etc.); fine dining (several places including the on-hill Ritz Carlton). Northstar does not have the steeps of Squaw, Kirkwood, Alpine or Sugar Bowl, but it has some good bump runs. Also it is a great area when it is storming and other mountains may not have complete operartion. Northstar is almost always completely open and has good tree skiing in powder or low visability.