My experience both working in a service industry and giving back to those working for me is that giving a tip gives back to you threefold, at least.
Booze, a gift card to a food store to buy some bits for Christmas dinner, a gift card to the local pub for a lunch is all good. I always offer coffee, tea etc as well. It's amazing what little things do for someone. I always go out of my way for people who go out of my way for me.