I make ladders and huts and sky-walks and such things using Popsicle sticks (I call them MES-Mouse Entertainment Systems) and Elmer's Glue...although if you don't have the patience of Elmer's, a hot-glue gun and a stick of hot glue works just as fine.
Here's a pic of one of my ten-gallons for the MES's:
You can also use TP rolls, egg cartons, construction paper that are cut into strips and made into chains (again, don't use tape to make the links on the chain, use hot glue or Elmer's), you can drape strips of paper towels over a bowl...wet the strips with water, like paper-mache, and let dry (this makes an excellent mouse-den...but they will shred it in a week), hammocks made of fleece are always a hit, as are tinker toys....
Balsa bird ladders are fun as are the little wooden boxes/baskets that gift assortments are packed in. They last a lot longer than cardboard. Egg cartons are a lot of fun. I'm thinking of setting up some kind of 'high wire' out of thick cotton cord and see is any of my meeces are acrobatic. They always climb onto the little handles in the plastic tank whenever I invert the lid during feeding/cleaning. I have thought for some time that those little jungle gym thingies that are made for birds would be a real hoot, but they're kind of pricey. Sheets of TP can be good short term fun. And of course even I enjoy toobs;; I have some of every type available and I enjoy finding new ways to configure them to fit in the plexiglas tanks.
I know the pix are a little hard to see but here are a couple of our cage set ups. I change them as often as I can and replace things that are soiled. cewed beyone use.
This one has a shed (it was used for weanling bucks and I find its best not to give them something completely enclosed) and walk way made of popcicle sticks. I have a tennis ball on a rope chew toy strung up with twist ties (OMG did they love that ball!) There is also a chain of plastic hearts (a party favor to make necklaces and stuff) they they can crawl up and through.
Thisone has an infant chain made of thick plastic that i found at a flea market for 25 cents. There is a rope and canvass dog chew toy that I put up with twist ties, it swings and rocks when they climb on it (this has been the bigest hit of all the toys!), I have whiffle balls that I cut large holes into hanging at differnt levels (they love to get into them to curl up and play king of the mountain on them) There is also another rope chew toy with a large knot in the middle. The knot provides a rest spot and when more than one climb about on it it swings and sways.
I dont think they can be seen in the pix but there are also cat toy jingle balls (mesh balls with a little bell in them) both hanging and onthe ground. They love these too! When I am not feeling very creative and for some of the older mice who dont fancy all the "toys" anymore I will also use branches (after baking them int he oven) and unusual rocks I find to give thema more natural feel. I like to place a slightly rough rock in areas they frequent as it helps to keep nails trimed.
I love looking at all thebird toys our local pet shops sell. I wish I could afford to buy tons of these but all the ones I think the mice would enjoy are just so expensive! I would love to take the ideas I have seen on a lot and build my own.