Above is the Visitor Center, (the former Buttrick Estate) and below is the side entrance, facing the gardens.
Old photo in frame, inside the Visitor Center, showing the house and gardens.
At the end of the gardens, there is an overlook, in which you will look down on the Concord River and North Bridge. A short trail will take you to both. Below are the river and bridge taken from the opposite direction.
In my opinion, the best time to visit is early in the morning, perhaps 30 minutes before 10 AM. You will be there before the bus loads of tourists or school children arrive. When the Visitor Center opens, you could be among the first to get inside.