Chew toys:

Rattie teeth don't stop growing; gnawing helps to maintain a good length. Chew toys need to be provided!

Leftovers:
--chicken bones
--steak bones
--pork chop bones
--rib bones

Other:
--Edible nylabones (potato, bacon, or fruit flavors are popular with rats. My girls hate the ham and cheese.)
--Greenies
--wood chews
--fruit-tree limbs (such as apple trees... these also make good climbing toys)

Climbing toys:

--the wire of the cage itself (including the roof!)
--bird perches
--ropes
--thick dowel rods or fruit-tree limbs
--placing hammocks or furniture and shelves to jump/climb from one to the next

Other Toys:

--Cat-toy balls with bells in them (no catnip!)
--small bottle with treat inside (try to get treat out!)
--paper towels, tissue, cloth to dig and chew through
--water dish (can get messy here... heh. Add some frozen peas ;) )