Grab a couple of cardboard boxes (Popcorn, Soda boxes, etc.) and toss them in for the rats to hide in. If you have the extra money, buy a couple of igloos and ferret tunnels. This will give them things to hide in to feel secure.
By the way, cat, bird, rabbit, ferret, etc. toys are perfect for rats. Many times I can find the cat toys (feather, jiggle balls, crinkle balls) for less than 50 cents. The rats seem to adore them and they are disposable so when you do a complete cage clean, you can toss them.