The Arnold Arboretum is a National Historic Landmark and a great place for plant lovers. There is a great collection of plants in the area, and it's fun to read about them as you're walking around with your pup. The area is split into 3 hills: Bussey Hill, Hemlock Hill, and Peter's Hill. It will take around 3 or 4 hours to make a loop around, including time for plant learning!
Peter's Hill is better known as the more dog friendly section of the park. Because of the Arboretum's popularity, it is important that you keep your dog leashed. You might get yelled at otherwise! Be sure to bring poop bags with you too, they aren't provided here.