Twice over the weekend, I have found an exhausted bumble bee on the floor. Both times, I popped a little blob of honey near them, their antennae pricked right up and I could actually see them drinking it.
Within 5 minutes, they both woke right up and flew off.
My children particularly enjoyed doing this, it is so nice to feel you can help the little things, I don't know how long it helps for but at least I tried.