How to keep an older mare healthy

I have a 20 y/o Arabian mare and was looking for ideas on how to keep her healthy. She lives and rides in a pasture and I have a small barn for when the weather is bad. I don't have an arena, but we do some arena work on the fencerail. I want streches, exercises, games, almost anything to help her stay happy and healthy. I don't use her to compete, so that is not an issue, as far as perfectly muscled or super shiney, but I want her to stay active and healthy for years to come.