This is a good activity to give your rats and requires only a tiny bit of effort. Take a few treats, pretty much anything works as long as it isn't liquidy or wet, and place them in the center of a stack of 2-3 paper towels. Bring up the ends of the paper towel to form a ball around the treats, then tie a string around it to keep it all together. Suspend the pinata from a higher place in the rat's cage so that the bottom of the pinata is only in their reach when they stand up. They will be able to smell the treats through the paper towel and will have fun tearing at the paper towels to get to them. If your rats are having trouble figuring it out, make a small hole for them to get started. Once they figure out there is food inside they should get it fairly quickly!
A good way to keep rats cool in the summer! In a dish which will not tip when stood upon, fill with an inch of water and add several frozen peas. The rats will have fun trying to fish the peas out of the water and it will help keep them cooled off in hotter weather. Another suggestion is to use a roller tray with the bottom portion filled with water so the rats can gradually wade in to the water instead of just dipping their noses.
If you don't mind the mess, a simple box of dirt can be fun for your rat, as it is a natural instinct to dig. Choose a sturdy dish that will withstand chewing (or one that can be thrown away if they chew it too much) and fill it with plain dirt that doesn't contain fertilizer. You can also use potted catgrass, which the rats can either chew on or have fun digging up. A box of dirt alone is fun for the rats to play around in, but you can make it more interesting by hiding 'buried treasure' such as treats that can withstand being buried or small toys for the rats to find. If you put the digging box in their cage, try to clean it up as best you can when they are finished so there isn't dirt all over their cage.
Something fun for your rats to do in or out of their cage is to give them a hard-boiled egg with the shell still on. Crack it open just enough so they can taste what is inside and place it in an area where things can get a bit messy. The rats will have fun tearing into it (they won't eat the shell) and finding the yolk inside. For an added challenge, don't crack open the egg, they will be puzzled trying to figure out how to get it open themselves, especially if they already know what is inside. You will likely still have to help them get it open, though!