A few things that haven't been mentioned:
Holding doors open for people with there hands full (or not full).
Helping somebody who is struggling to get their stroller on the bus.
Buy a few extra "glow in the dark" ropes while waiting for the parade or fireworks, and give them to children who don't have one. If you really want to go all out, you can even get an extra one or two of the other "light toys". Most kids would love to have them, but some parents don't want to buy them because they are over-priced. Something I just thought of: I believe you can buy the light ropes/sticks in party stores. Why not buy a few and bring them into the park to give out? It would be cheaper, and Disney won't say anything as long as you don't bring a hundred of them. (pausing while I add this to my "to do" list before our trip)
And as others have said, smile at CMs (including security, the street sweepers, etc) and be nice. If you bump into somebody, be the first to say your sorry (with sincerity). Of course, the fact that you are asking the question probably means you already do this type of stuff, but you never know!
If you have a particularly good encounter with a CM, note their name and when/where it happened and send it Guest Relations after you get back (or while you're there if you want). For characters, I've heard that if you just note the time/place/character name and send it in, they make sure that CM gets credit.
If you hear a guest say the can't find something, or don't understand something, and you can help, go for it.