There’s a Kmart near me that I sometimes walk around when it’s raining outside. Even though it’s not endless like outside, the tall isles block your sight lines so you can wander for a while.

Walking in the cold and/or rain is also quite nice.

How big is your office?

My home office is large enough to walk in circles (I have heard that my grandfather used to walk in circles when thinking, it's probably genetic :P). When I'm in an office building, well these usually extend by a few tens of meters in at least some direction.