Play Tag, Steal their fruit ( My favorite ), Go fishing with eachother, Trade items, Host contests, Machinima, Talk and chat, visit their neighbors, and Finally Chill at their house.
My friends and I had a fishing contest today where we were to catch 10 fish, and sell them, the one making the most money being the winner. It was good times.
Yup basically everything posted above. You just have to be a bit creative. Even talking to people, just letting eachother know what you guys have accomplished in the game, is fun.