From Sendai to Matsushima is nice. There is a free campground on the north side of the bay I've been meaning to go to. Shichigahama is nice to. It's a little hilly bu the ride along the coast is beautiful and there are several beaches along the way where you can just pitch a tent for night if you want. You can also go from Sendai down towards Akayu. There are a couple campgrounds down there, it is about a 50 km ride and it is pretty mountainous but it's fun nonetheless. There is a nice onsen about 20 km from Sendai in Izumigatake that's worth visiting. It's a nice ride because it's all uphill on the way there but after you've had a couple hours in the onsen it's nice to have an easy ride back.