DIY Rat toys. Most things on the list can be made in a larger scale for ferrets (Note: the cakes are not ferret-friendly, do not feed fruit, or bread, to ferrets).:
What a great rat playground accessory ! Looks like fun for all and when they get bored....instant chew toy !
Two 12x6" shelves with play tube rat,degus,chipmunk cage | eBay
Proud of your hamsters cage - Page 381 - Supplies & Accessories - Hamster Hideout Forum
Project IKEA - Platform/Level - Page 26 - Hamster Central
Good for small pets. Especially like the stairs made with twiggy bits attached to a strip of wood. I have seen twigs or chunky wood dowels like those used as chews or bridges in the pet store. --Meggie