Tunnels, Hammocks, Hideaways, etc.
Rats are very intelligent and get bored very easily. To keep them occupied you should have plenty of hideaways and things to climb in the cage. Their should be at least one or two hideaways or beds per rat. A good idea is to have at least one or two plastic or wood houses, and some tunnels, hammocks, and other sleepers. Make sure that everything is big enough for your rats. And also make sure that more than one rat can fit inside each item to prevent fights.
SuperPet makes a lot of good tunnels, hammocks, and hideaways such as these:
These are all great for your rat. The plastic igloo can be washed, and the hammocks and tunnels can be washed in the washing machine. Most of these items and others like them can generally be found in either the small pet section or the ferrets section of most pet stores.
Another good brand of hammocks is Marshall Pets. They are meant for ferrets but work great for rats too!
You can also find plastic tunnels and some cardboard tunnels which are great too. Just make sure the rats don't chew the plastic. Chewing the cardboard is fine.
Tip: You will want to wash the hammocks, tunnels, and hideaways regularly too keep your rats environment clean. For fabric hammocks and tunnels, a good idea is to have extras so you can always have some in the cage even when others are being washed.