Making a tunnel playground for your gerbils is very fun and easy, and your gerbils will think that it's fun, too! Also, it doesn't cost that much!

[edit] Steps

When you're finished with toilet paper and paper towel cardboard tubes, save them!! You'll need around 3-5 of them to make a tunnel playground.
Once you have enough tubes, start making a tunnel system. To join tubes together, use scissors to cut 4 little slits around the very end of the tube, by the hole. Put the other tube in between these slits to join them. If it isn't fastened on tight enough, make the slits deeper. Make the system however long you want, but it has to fit inside the cage.
Cut gerbil-sized holes every here and there, so they can come in and out.
Place your tunnel playground inside their cage and watch the excitement!

[edit] Tips

To make an underground tunnel playground, connect a system of tunnels that is the longest it can be but still fit inside the cage. Cut a hole in the very center one. Put another tube going upwards inside the hole. The cage should be empty when you put the underground tunnel system in. Place the tunnel playground anywhere in their cage, but make sure the extra tube is going upwards. Put their bedding in and the rest of their supplies, but make sure the tube going up is accessible. When your gerbils go in this tube, they will go under their bedding. Don't worry about them having to climb back up-- the holes you cut will enable them to crawl out.
If you don't have gerbils, you can donate your tunnel playground to animal shelters and pet stores.
You can also sell your tunnel playgrounds to people who do have gerbils. It's an easy and fun way to make money!

Do not use the underground tunnel playground if you have gerbil pups or if you are breeding!!!
Do not use tape to hold the tubes together! Gerbils do chew on the tubes, and they cannot digest tape!
Don't forget to clean your gerbils' playground once in a while, otherwise it will stink!