Gerbils are very active and inquisitive and therefore need a lot of stimulation to keep them occupied and prevent gerbil boredom. They need to be able to interact with each other and their environment to keep them happy and interested. This is why it is important that you keep your gerbils in pairs or small groups and why it is important that their environment provides for all their needs.
Housing that gives your gerbils the opportunity to dig will provide much of you gerbils needs as digging is rather fundamental for gerbils. You can make homemade gerbil toys, or buy gerbil toys from pet shops, and place them in your gerbils cage to add interest. Gerbil toys might include items such as toilet roll tubes, climbing frames, or you could be ambitious and make fun structures out of lego blocks! A good pet shop will also provide lots of gerbil toy ideas. Gerbils also spend a lot of time nibbling things so don't expect anything you put in their cage to last very long! Providing you gerbils with a gnawing block will help them keep their teeth from overgrowing but will also hopefully prevent them from destroying other objects in their cage quite so quickly.
Occasionally you may like to give your gerbils an opportunity to run around a room in the house. This will also help to prevent gerbil boredom by creating a change of scene for them. Be careful to ensure the room is safe for them to run around in and make sure you ask other people in the house to knock before entering the room in case one of your gerbils gets hurt by an opening door.
Alternatively, some pet shops sell gerbil exercise pens which consist of wire panels that can be attached together to create a pen within a room of your house. The wire bars are vertical so that gerbils can't climb up and over the panels. You should inlcude objects and gerbil toys to make the pen more exciting. You should only allow your gerbils to run around in their pen under your supervision. It's also a great chance for you to interact a little more with your gerbils for short periods, before they go back in their cage to continue their digging!