Birthday Ideas Anyone?

My sister is throwing a birthday party for my nieces.(Lizzy & Heather) They'll be turning 6 on the 26th. They live in a town home and their neighbors are like seventy and hate kids. I don't know why. But anyway we're thinking about having it at my husband and my house. The only problem is... we have no idea what theme we should use. She asked me to help. We have a pretty decent sized pool.