First and foremost, toys for your gerbils must be safe. Gerbils have a habit of sticking their heads into things to explore, so make sure any toys (especially those with holes cut in them) don't have spaces in which your gerbil could get his or her head stuck. Make sure your gerbils aren't able to splinter off sharp pieces of plastic or other materials and swallow them.
Remove any plastic toys once the gerbils start chewing on them. Also make sure they are just shredding cardboard, rather than eating it. Make sure they are no parts or strings on toys that could trap a gerbils toes, legs, or tail.
Many gerbils like running on wheels. Any wheels should have a solid surface and have a solid back wall. Their tails and other body parts can get caught in and injured on wheels with rungs or cross-supports. A solid wheel like a Silent Spinner or Wodent Wheel is a good choice.
Get a medium-sized wheel, large enough that the gerbils back can be held relatively straight while running. Not all gerbils like wheels though, so don't be concerned about gerbils who don't use them.
Wooden hides and houses (untreated wood) make good choices, though small cardboard boxes will be a hit too. They will quickly shred cardboard though, so these will have to be replaced often (choose cardboard with as little ink as possible). Wood will be chewed as well, so while they will last a lot longer than cardboard, you can plan on replacing wooden houses after a while, too.
You can sometimes find hollowed out logs at pet stores, either small logs in the small pet section or perhaps slightly larger half logs in the reptile section. A half coconut shell and ceramic or clay plant pots placed on their sides can be used this way too
Gerbils love to chew. A cardboard tube from a toilet paper roll or paper towel roll is one of a gerbil's favorite things. Some people have expressed concern about the types of glue used in these -- and now colorful safe (and a bit more sturdy and long-lasting) cardboard tubes are made for pets to play with.
Look for Totally Chewbular Tubes, Chubes, and Critter Caves, along with others. Twin Squeaks also has a wonderful suggestion -- just take old file folders, lay them flat and cut them into smaller sections, then roll up tightly and hold for a bit. When you let them go, they loosen but stay more or less rolled, and your gerbils can have a ball with them. Even a small brown paper lunch bag will provide hiding, play, and shredding possibilities!
Any untreated wood (vegetable dyed is okay) items make great chew toys too. You can buy them or make your own out of untreated wood scraps. Branches can be offered for chewing too - try apple or willow (avoid any evergreen tree branches). You can also check your pet store for things like cholla, driftwood, and other wood such as grapevine (which you might in the reptile section) which make good climbing toys.
There are also lots of wicker and natural grass and plant fiber tunnels, balls, and huts out there (especially marketed for rabbits), and these are also great for gnawing.
Climbing Toys and Tubes
Gerbils will climb on just about anything, such as wooden houses. You can also buy wooden ladders (check the bird section if they aren't in the small pet section), see-saws and other wooden accessories. Plastic climbing tubes (like those for hamsters) and accessories can be used too, though you need to be extra careful that your gerbils are not splintering off bits of plastic and swallowing them.
Plastic toys and tubes are best offered only when you are there to keep an eye on things. You can also try PVC pipe sections from the plumbing section of the hardware store, as PVC tends to be a little more resistant to chewing than plastic tubes made for rodents. Relatively new on the market are dyed bamboo tubes, and these are said to be tougher and longer lasting as well. You can also find ceramic tubes (check the aquarium section), and these have the advantage of being virtually indestructible.
Give you gerbils a heavy ceramic or glass dish with a couple of inches of chinchilla dust in it (chinchilla dust bath available at pet stores). Your gerbils will most likely love rolling and digging around in the dust, and in the process get their fur cleaned and conditioned.
Gerbils love to dig. Try to make at least a part of their cage very deep with substrate. You can also place wood shelters (such as under the shavings so the gerbils can have little "underground" burrows to sleep in. Adding timothy hay to the bedding will help the bedding hold tunnels better. You can also put ceramic tubes (check the fish section) under the substrate for burrows.