When the ground is dry and the weather is good, I like to take as many bunnies out side as possible for a chance to stretch their legs a little bit. The pen pictured is my smallest pen, most of my pens are 4X4 or 8X8. I have one pen that is 12X12 for multiple bunnies. I also use, used play pens and flip them up side down.
Here is a Video of one of our bunnies hopping freely in our back yard for exercise.