I'll assume you're interested in non-straight, non-flat trails as well since you listed the town forests. There are some interesting loops you can find out there in state forests, e.g.
Harold Parker
Great Brook Farm
Middlesex Fells (fire roads)
Minuteman National Park (one of my favorites)
Reformatory Branch
Billerica Town Forest (stay away, IMHO, it's not very nice there)
Rock Meadow in Belmont has a number of flat trails and you could possibly even bike the
Western Greenway - I was just out there this weekend, and this place has lots of flat, wide trails. Saw a number of bikers out there.
(ones I haven't been to but want to check out some day)
Ware River Rail Trail
Topsfield Linear Common
Methuen Rail Trail
Google Maps with the cycling layer is a good way to find places to explore (look for brown):
https://www.google.com/maps/@42.6916.../data=!5m1!1e3
OpenCycle map (OpenStreet with the Cycling layer):
OpenStreetMap
NEMBA (read descriptions first, as it's MTB-oriented):
massachusetts | NEMBA