How tall and wide of a tree is acceptable without interfering with the ditch itself, utilities, walkways etc? Is it a residential, business or a more rural or park like situation? I would guess hard mast, soft mast and berry producing trees and shrubs which would benefit wildlife and be used by people as well. Oaks, pecan, hickory, hazelnut, apples, crabapples, pears, american plum, nannyberry, serviceberry, elderberry and the list goes on. Variety is the spice of life.
The one concern I would have is if you are more rural and have deer in the area. Using cages or tubes to protect young trees from browse can be as expensive as the plants themselves. Having kids involved in the community is awesome. They will remember what and where they planted until they are old and grey.