Instead of buying a fun sprinkler to keep cool, reuse a 1 liter bottle! All you have to do is poke some different sized holes at or towards the bottom! Then, you take the hose and hang it so the part where the water comes out is facing down. Put the hose inside the bottle(tape it a little to help it stay), turn the hose on, and enjoy the soggy fun!
You could also build something fun! If you have a small, sturdy tree with low branches, try building a swing, or a fort! With some leftover rope(stong rope), and some wood, you could make a swing! Make sure it can hold a reasonable amount of weight, though. You don't want to find out it couldn't hold more than 2 pounds after you get on it! Also, you could make a fort. Start with a roof, and let you imagination take you from there! Watch out for when it rains! If you have things like a doll or some tacked up pictures in there, take them down so they don't get soaked(that's what the sprinkler is for)!
Lastly, if you want to get some energy out, and was told you couldn't do it inside, make an obstacle course! If you have some hoolahoops, you could make someone build a castle, or jump in and out of them! Then, if you have some chairs and a big blanket, you could crawl through a tunnel! With very little materials, you can still make a great obstacle course!
So, I hope these ideas help entertain you over the summer!