Because of their natural curiosity, gerbils are usually interested in toys. There are many options available at your local pet store as well as online. Every gerbil cage should include a wheel so they can get plenty of exercise. I really like the wheel we got. It’s very similar to this one available at Amazon:
The best way to mount the wheel is either from a wall or the lid because it allows gerbils to have more space on the floor and prevents the wheel from tipping or getting partially buried in wood chips. I modified the wheel pictured to wrap around the side of our glass aquarium by bending the hooks with a pair of pliers. My wife likes to oil the wheels or take them out at night so the gerbils don’t keep the entire house awake with their running.
My daughter also really wanted to get an exercise ball. So after visiting the pet store she ended up purchasing the Ware exercise ball (left) because it was slightly less expensive. The ball is two-toned, which looks a little more interesting than the solid color balls. It has two halves that lock together. Unfortunately my two-year-old son dropped it, and the plastic chipped. I’ve used Kaytee in the past and have had good luck with that brand.