The Garden Igloo Kit. in a few different versions. could be a greenhouse +/or alternative to tent accommodations to house a few more summer retreatants. not particularly cost effective though, in my humble view.
Outdoor shower/water day for kids.Could easily be made with hollow box uprights, run the water tube through one and the top hollow box with a PVC tube that you drill holes in for the water to run out. Faux paint it to look like copper, really pretty easy.