Balls are one of my son's favorite things, so I am thinking of using them as a theme for his second birthday.  Invitations could be written on small balls and handed out, or rolled up in small balls that open. Guests could be asked to bring a food to share that is round. To keep the kids busy I could set up various ball games (maybe borrow someone's kiddy basketball hoop, and a T-ball set up). Another fun activity would be to rent one of those ball cages where the kids could swim around in a pool full of colorful balls.  Decorations could be inflated balls hung around.  Party favors could include little hand held ball games, and of course balls. This theme would be easy to do in a backyard or at the park.