well for some one who needs ideas, you sure have a lot of them
have a bikkini party. that way you can have a slip n slide, or sprinklers or a hose fight or what ever.
just have a dry place to hang out (maybe protected by a tarp?) and plenty of drinks and snacks. and no cake, just ice cream.
or for in doors, have an indoor bikkini party. i don't know what you'd do, but it's worth a try. maybe just dance and play truth or dare.