Right now they are feasting on apple, pear, and plum blossoms. Plus trees are blooming - maples, willow, etc. Over the summer they will have dandelions and other wildflowers. I'm planning on planting more bee balm, calendula, snapdragons, etc. Adding lots of flowers.
While I don't care for buckwheat honey, I'm planning on planting a stand of buckwheat just for the bees - I'll harvest the honey (IF I get to this year) just before the buckwheat blooms. That way, the bees can have that honey for their over-winter stores.