Gerbils enjoy wheels a lot. You should be careful with the wheel you pick for your gerbil. Metal or plastic rung wheels are not safe for gerbils because they can catch a paw or their tail in it and break it. Plastic solid wheels are good for them, but they tend them to chew them. I like the metal mesh wheels, as they are safe for their feet/tails and almost impossible to destroy.
Another good entertainment for gerbils is litter. If you provide a deep layer of litter they will happily dig around.
Wood toys are also good. Of course, they will get chew to bits, but gerbils need to chew to control the growth of their incisive teeth, so at least a small wooden toy is a must in a gerbil cage.
You can also provide cardboard for your gerbils to chew. Paper towel and toilet paper rolls are good, also cereal boxes. Avoid printed cardboard that didn't had human food in it (like a printer ink box, for example) as it can be toxic. Also avoid boxes of frozen food, because they are covered in a layer of wax which can also be toxic.
Last, avoid plastic toys. They get chewed to bits in no time and can be toxic if ingested.