We used our extra snack credits and counter service credits to treat people behind us in line (or, in the case of ice cream stands, nearby kids who looked like they were wearing out -- after checking with their parents, of course.) It got a massive smile every time! We also did the glow-bracelet thing and will do that again this year -- our kids loved sharing them with the kids nearby while waiting for spectro/fireworks to start. Sharing fastpasses really seemed to be well received, too; if we had ones we knew we weren't going to use, we'd go to the fastpass area or the end of the regular line and offer them to someone. Once I had someone ask me what they were, though: obviously not a DISer!
I agree that taking pics for families is an awesome way to spread pixie dust, too. Someone did that for us and took the only pic of us ALL together that we got on the entire trip!
And finally, one of my favorites -- on the night we arrived, it was absolutely pouring rain, and someone's early-teenage son (I think they were waiting for their dad to come around with their car) ran out into the rain as we came in from the parking lot and offered to help me carry our luggage in to the sheltered area over the sidewalk, where his mom and little sister were waiting. He made 3 or 4 trips out into the rain until we had it all in! I was so amazed by his kindness, and it really made an impression on me -- so now I try to pass the favor along when I see other families struggling with loads of luggage!