B.R.
You could set up a slip and slide and other fun water toys etc.
As fars as games try hotpotoes with water filled balloons and water gun hot games. Make a target and see who hits it.
We are doing the same for my sons bday but he is only 3! But I am even setting up a small pool for some babies that may come so they can have a small pool area with some small water toys from the dollar store.