I got my cats a Cat Cottage from Foster and Smith for about $25.

It's basically just a big carbboard box with 4 levels with holes so
they can climb from floor to floor. It also has openings in the sides
for doors and windows that they can go in and out or reach through and
play.

They race around the house and through the cottage and sometimes I hear
hear them banging around in there chasing each up and down. They like
to reach through the windows and bat at each other and then they will
take off, chase each other around and run back into the box again.
Sometime they just go in and sleep.

It's printed on the outside to look like a house. It sure looks a lot
nicer sitting there in the living room than the empty case of toilet
paper that they have been playing in for the past year.

It's the best thing I ever got for them. They love it.