[Crystalroborovski]

The bathroom is a risky place for free ranging. There are many places that she could squeeze in and behind, such as behind the toilet or sink. Also take into consideration that many surfaces, even sometimes the floor, could have been cleaned with chemical cleaners such as bleach. The floor may also have human waste on it (gross). Given these reasons the bathroom really isn't the best place for free ranging.
As amity mentioned you could use something as simple as a cardboard box and spend little to no money on a playpen.

Free ranging can be fun if done correctly but I wouldn't recommend doing it in a bathroom.