Rats are very intelligent. They need constant mental and physical stimulation to keep happy and healthy. There's tons of things just lying around the home that can entertain your rats. One man's trash is a rat's treasure ;)

Paper towels
Paper towel rolls
Tissues/Toilet paper
Toilet paper rolls
Tissue boxes (with tissues still in it)
Cardboard boxes
Homemade hammocks
Homemade tunnel hammocks
Homemade pinatas (food wrapped in cloth hung from top of cage)
Digging box (plant rat-safe plants in an aquarium. cat grass and fruits/veggies are great)
Wading pool (put a couple inches of water in a pan. add smooth rocks and frozen peas)
PVC pipe
Old clothes (socks, shirts, towels, etc.)
Blanket
Stuffed animal
Ratty Popsicle (take a small dish. cut up lots of different fruits/veggies and put into dish. fill with water and freeze.)

Things at the store:
Edible huts/logs
Plastic igloos
Wooden hide-aways
Bird toys
Cat toys
Baby toys