Something which takes some time to train up but is good exercise and tons of fun: before Babybartfast was born, I was with a K9 search-and-rescue unit. My dog and I learned how to search for lost people in several local environments (the woods, swamp, mountainside, fields, night/day, indoors, etc.) and were working toward cadaver and water searches when I ended up leaving the team. It was really interesting and required very little medical training - on a real search you're pretty much always leading a small team of volunteers, usually volunteer fire and rescue, so your job is just to read and interpret your dog's signals and help the medical people get to the missing person. You don't have to have a dog to do this, either - we had some people on the team who stayed in the main base on searches and organized who was searching where, oversaw the radios, acted as liaisons with the other departments, kept track of the topographical maps, etc. The other fun aspect of the whole thing was going to schools/scout troops/summer programs and talking to kids about things like what to do if you're being chased by a strange dog, how to avoid getting bit, what to do if you get lost in the woods, etc.
There may not be a search team convenient to where you live - there are three in my state and all three are in my city - but it's worth a look. You get a lot of exercise tramping around outdoors! Even if you don't want to actually join, search teams LOVE to have volunteers come out and hide for the dogs. It helps the dogs to not search for the same people (with the same scents) over and over again, and it helps socialize them. We occasionally had (older) scout troops and teen volunteers come out and help at practice, and the dogs loved the attention!