Rats do not require chews as they can grind their teeth down by themselves. However, gnawing and chewing is a great natural activity for them that should be encouraged!
Bones, raw or cooked, make great chewing toys that are very nutritious. Raw bones should be removed within a few hours to prevent spoilage, but cooked bones can be left in the cage until they've been completely stripped of excess tissue and marrow.
Rib bones are a favorite in my mischief.