Best Kid Hikes?

Hey Everybody, I've got two kids (4 and 3) with a third on the way. The kids love being outside and they seem to enjoy hiking as long as the trail isn't too long or intense. So I'm looking for suggestions for some good day hikes for kids. Also looking for advice on how to help kids enjoy hikes when their enthusiasm begins to wane. Any success stories? We've done Sturtevant Falls and Switzer Falls and the kids had a good time on those (though my back was sore from carrying the littlest one for some stretches).