Teen water fight ideas for 2 people?

Me and my bestfriend are both teens and are gunna have a sleepover. The problem is, my parents have to work so we have no transportation anywhere. We decided to have a water fight. We want it to stay fun but we cant invite anyone else. (except my little sister) . We have balloons, buckets, sponges,guns, etc. but we dont know what to do where it wont get boring. Anyone have any game ideas? Thanks so much!