Everybody's suggestions have been great. I'd like to add an important source of bee forage: flowering trees and shrubs!
Some important nectar and pollen sources include: red maple, lilac, rose-of-sharon or rose mallow, willow, dogwood, fruit trees, tulip poplar, black locust, flowering brambles like blackberry and raspberry, azaleas, holly, and more.
It's also nice to let flowering weeds, like common chickweed, purple deadnettle, henbit deadnettle, gill-over-the-ground (ground ivy), yellow woodsorrell, and dandelions grow freely in your yard. Bees love these.