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This new all wood bridge was rebuilt in 2010, and is one of only three covered bridges on public Massachusetts roads that are open to vehicular traffic. It's also the closest to the greater Boston area. The bridge is designed for cars, but there are pedestrian walkways available on both sides.
Bancroft's castle
Built in 1906 and ruined in 1932, Bancroft's castle is an impressive structure with a tower and overgrown stone walls. Public access is free and unrestricted, although it can be hard to find parking on nice days. There are also some good views of the area from the top of Gibbet Hill. Note that despite how it looks on the map, you cannot drive directly to the castle; park on the north side of Lowell Rd. near the stone pillars that mark the trailhead, and there is a short but steep trail up to the castle.
Groton Historical Society
A 12.5-mile paved bike and walking path on the old Hollis branch of the Boston and Maine Railroad.The rail trail Starts in Ayer, MA and runs through the towns of Groton, Pepperell, and reaches its northern point in Nashua, NH.