Our European outdoor activity adventures, besides lots of hiking and trail walking, have included biking and paddling.
You didn’t mention bicycling, but our first adventure was in Belgium itself, a country that’s as much about cycling as it is chocolate, beer, and deep fried potatoes. There’s an amazing support system, with well marked, paved bike trails all over the western, Flanders part of the country. I don’t know about the eastern part of the country, but I’d guess it’s extremely bike friendly, as well. Flanders is pretty flat, so 6-speed bikes were well suited for the terrain. Electric e-bikes have apparently gotten popular now (I don’t personally have experience with any), but assistance from an electric motor could make it even easier to go farther or faster. We rented bikes in Brussels, took them by train to Bruges, then biked over several days as far south as Ypres, before turning towards Brussels. We got to Brussels by train at the end, turning in the bikes. We brought a tent and sleeping bags, and were ready to camp, but wound up doing that just one night, staying at B&B’s the rest of the time. You could conceivably stay at your home exchange house, and do out-and-back spoke trips from there. Beers after a pedal are even more satisfying!
Next door to Belgium, in France, we’ve canoed the Dordogne River, with lots of advice from Rick Steves about where and how to rent, put in, and take the boats out, plus sights along the way. Then, on the west coast of Brittany, we did a sea kayaking trip, with an overnight on the uninhabited Isle de Groix. Our guides set up the tents, cooked an amazing dinner, and made sure we got there and back safely over the 2-day adventure, crossing open water in the sea, seeing birds, and having a grand time. You do get a bit of a workout, especially if there’s a breeze, but the adventure was amazing!