Here's a cheap idea that provides enrichment for your ferrets and gives them digging time when they can't go outside...
a Dig Box!
You can buy a plastic container like this for about $5 at Walmart and add some sand, dirt, soil, dry beans, or rice (but NOT instant rice- which could be harmful if ingested).
Here's some pictures of Slinky (DEW) and Weasel (dark sable) in their dig box:
I actually used old sand from an aquarium I took down, but you can use any substrate listed above.
I orginally got the idea from the Holistic Ferret Forum- here's a few dig box discussions for more ideas/information: