ShelterLogic 6 x 6 ft. Shed-in-a-Box: Very affordable extra storage. I have a small backyard and I can just about fir it in. I didn't do anything fancy for a floor I just used a very large tarp that I attached to the top side beam. Putting a lot of dehumidifying crystals in it seems to keep things OK.
Make fishing boats out of cardboard boxes, lay down a blue tarp to make it look like water, use a sand bucket to put the fish in and use those play fishing poles with fish that you get in any toy section at any large super store