Bruce Freeman Rail Trail( from Sudbury over Concord, Acton (behind Teamworks and Donelan's, over Nara Park) to Carlisle, Westford, Chelmsford, and Lowell)
Assabet River Rail Trail (starting behind the South Acton railway station and going towards Maynard, Stow, Hudson, and Marlborough).
An overview of all bike trails in Massachusetts is here or here.
Other town/school owned facilities:
Elm Street, Acton (two courts-can be reserved via Acton Facilities)
61 Canterbury Hill Rd Acton (can be reserved via Acton Facilities)
Acton: Rental plots are available through the town for a small fee (higher for non-Acton residents), 1 plot per household.
Boxborough has community gardens at Flerra Meadows (flerragarden@gmail.com) and on Middle Road (middleroadgarden@gmail.com )
and there are many local private fitness centers.
Ice skating on outdoor ponds or lakes (eg Nara Park, Ice House Pond at 120 Concord Rd Acton) might be possible at times however not recommended by the town because of safety, they advise skaters to use indoor or outdoor man-made rinks (there are indoor ice rinks in Boxborough and Concord).
Acton Canoe Launch: 65 Powdermill road, Kayaks and Canoes only.
Lowell Road Boat Launch, Concord: Concord River, all watercraft
T.J. O’Grady Memorial Skate Park, Hayward Rd, Acton, Skateboarding and rollerskating.
Nara Park Acton
Maynard Country Club
Steele Farm Boxborough
See more about youth and adult leagues in the Children Age 5-18 and Events & Recreation sections.
You can rewatch some local sports games online on Acton TV.
Swimming pools open swim and swim lessons:
Concord Beede Center (with kiddie pool and diving pool)
Swimming ponds and swim lessons:
Nara Park Acton
Long Lake Beach Littleton
Elm Street Acton (pickleball lines/nets available, online free reservation needed)
Liberty Field Boxborough pickleball + tennis
AB High School courts, Acton (no reservations, tennis only)
Jr High school courts at Charter Rd/Mass Avenue lights in Acton (no reservations, tennis only)