Lava Ledges from SuperPet, marketed toward Chinchillas, but also excellent for rats.
Lava Ledges are available at most pet stores and also from Amazon. They screw onto any cage wall and can be arranged in a stagger pattern to make an interesting climbing wall that can be changed up as often as you like. They come in a variety of colors to brighten up your cage.
Our rats love to climb on them: The young energetic girls especially, but I have occasionally found bigger and slower Pirate up on a lava ledge placed randomly at the top of the cage and wondered how he made his way up there.
They are supposed to be safe to chew, and it is a good thing, because our rats love to chew on them - to no ill effect so far. The rough surface is also ideal for helping to file down sharp nails. We like to place a lava ledge under each water bottle so that the rats step on them every time they take a drink, helping to wear down the nails.
One flaw is that with time, the screw attached to the pumice loosens and the ledge is no longer held firmly in position, but can rotate loosely around the screw. I usually move the loose ledges under the water bottles, close to the cage bottom, so that the bottom prevents the ledge from turning too much, and so that I don't have to worry about a rat falling off the ledge if it turns.