This is definitely not an original idea, and I'm sure many of you have done it in your own homes, but we've been having so much fun with box ramps that I thought I'd post about it anyway.
We just took a flattened box, leaned it up against the couch and let the kids roll cars and balls down it.
One morning Daddy got creative and set up a ramp in the kitchen that ended with a tin pan full of water for "washing" the cars. The kids loved it!