Shoe Thief!

Every time one of them tried to climb (backwards) up the slide the other one would slide down and stop her progress. It was a great improvised game that entertained them for quite a while. As it progressed the game became more complicated and the list of rules more lenghty. It finally collapsed under it's own weight but by then another different/similar activity had taken it's place and it was a fun afternoon at the park for everyone.