The only limit is your own imagination. Here are some suggestions
For toddlers (5 ft. dia.)
For teens (10 ft. dia.)
Portable hen pen
Move it around to keep from wrecking the grass.
The hens love the new turf. You can add components and make it a hen house.
A Sun and Watering Station
If you have a lot of plants that need a little outside time.
A Kids water fun assembly
Build it over a tarp on a slope and create a slide.
A small painted dome (any color) with holes to create spray patterns.
It also will aerate water if you have fish. You will need a pump and fittings.
A Green House
Use your imagination for covers.
Plastic sheeting to keep things warm.
Netting to keep birds off.
6X6 wire mesh and chicken wire to keep out raccoons and critters.
A tarp to protect from hail or frost.
A tent full of pillows and electric lanterns with colored filters, anywhere you choose.
SEE SOME PHOTOS BELOW