Combine the maple syrup, soy sauce, and apple cider vinegar in a ziplock bag large enough to hold the tempeh.
Cut the tempeh into 4 slices (see image). Place them in the ziplock with the marinade. Make sure the tempeh slices are evenly coated, and allow them to marinate at least 30 minutes.
Grill the tempeh over your campfire, or you can cook them in a cast iron skillet with a touch of oil. Cook 2-3 minutes on each side.