Our bid days are required by Panhellenic to be low-key: we're not allowed to go off campus, and all events start at 3pm when the girls receive their bids, and must end by 8pm. We normally order pizza and have a cake, the girls get gifts from their Heart Sisters (or whatever your org calls them), and we play little getting to know you games. This past week on our bid day our VP-New Member Ed wrote down all these random questions, like "what's your favorite color?" "have you ever been out of the country?", etc., and everyone had to take one and answer it. Our chapter has just over 50 women, so it works well. We played games like Catchphrase and stuff after that. One year we made the floor of our CR (we don't have houses, so we do everything in our CR) into a giant Twister board, which was also really fun.
We also have time for formal pictures, so if you don't do that, you could always take the time to take pictures of everyone by class.
On informal recruitment bid day, which has less restrictions that the one after formal, we normally do the same types of things and then go see a "chick flick" together (make sure the new members are paid for by the sisters). Nothing like 50 girls in matching t-shirts taking over a movie theatre
One year we actually went to the drive-in movie theatre in our town.