PVC pipes...
there fairly cheap (get the 3" at the smallest 4" diameters are the best)
VERY easy to clean (throw em in the dishwasher or just simply clean in soap and water in the sink, and there usually too thick for most rats to chew on...(get the hard white plastic not the flexible black rubber)
you can easily attatch them to the sides and top of your cage with cable ties too! i would probably put a 3 way tee and cabletie it to the roof of your cage (clip the cable ties when cleaning time comes, this will give your kids a cool high vantage point) you can then hang a hammock underneath for fun acess...
long straight ones anc curved ones sable tied to the sides make cool tunnel ramps and hidey holes too!
ive found you can never had too many hammocks lol.
plantpots are another favorite, plastic of course to prevent absorbtion...
those plastic hamster run around balls, take the top off and hang them in the cage fill with some shredded fleece, my girls LOVE these, when cabletied its one of their favorite places to actually sleep, when hung from chains (never more than a few inches above a solid level) they LOVE to play with it and jump in it and let it swing!
small wicker hanging baskets are also alot of fun