What should I do about neighbors barking dog?

I know what I should probably do, but am avoiding it. My neighbors got a puppy last year and I guess the novelty wore off. The dog now stays in the backyard all by himself. Nobody plays with him and I don't think he has many toys (I've peeked over the fence) He constantly chases his bowl across the patio and barks indefinitely. I know he is just lonely, I have even offered to the kids to bring him over to play with my dogs. I have tossed some rawhide chews over the fence just to keep him busy so he quiets down. I don't want to call the authorities on them because I don't want to cause problems (plus I wouldn't want them to do it to me, even though my dogs don't bark). Should I leave a note on their door? Please suggest anything, I know I am eventually going to have to suck it up and say something, I just don't know how. Help before this dog drives me to the nut house! :)