We visited Garden in the Woods this weekend and had a great time.  It is a huge collection of native plants and flowers in a forest setting, with a winding path that’s about a mile long.  Many flowers are now in bloom so it’s really colorful.  My daughter is four and had no trouble walking the whole way, because there are places to sit and rest.  They give you a list of things for kids to find along the way, and she had a ball doing that.  I suggest wearing sneakers or walking shoes, as the path can be bumpy, and wear insect repellent.