The Piste des Forts, or fortress cycle trail, covers 85 km in France and Germany and takes you round the 19 fortresses of the "Fortress Belt."
I will take you on a bike tour that covers parts of the trail (20-30 km as per your wishes) in both countries. You will see beautiful scenery and enjoy the fresh air of the countryside in your lungs.
I regularly bike on this trail and would be very happy to take you along. If you don't have a bike with you, you can use a Vélhop-bike which is part of the bike sharing system in Strasbourg.