I like the idea of new trees being planted however that may be. Also a water feature is a great idea to promote a more calming atmosphere.
I agree that trees are what draws people (Lots of urban planning evidence for this) - restful beauty & shade. If large stature trees will not thrive because the area is overtop the parking garage, there are lots of native medium & small stature trees and shrubs that can fulfill the same purpose: Serviceberry being one.
The water feature doesn't have to be big- I recall the small water fountain on the east wall of this square. (Is it still there?) It was wildly popular because it attracted the kids, created a "splash factor" yet didn't take up a lot of room or require huge infrastructure.
Personally, I think a splashpad would be unnecessary - sometimes an "unstructured" playplace has better play value.