@Wuzzy The single greatest thing I've done on OSM was adding all the field numbers at the local soccer pitch to the map.

I only did it so my wife and I wouldn't get lost when the kids had games, but I was able to get a few parents to sign up and help map our city because it was so helpful.

And if I can offer one piece of advice to people considering contributing it's this: if you only want to add one kind of feature, that's totally okay. Someone else can and will fill in the rest.