Love the Minuteman Bikeway! No matter what level you are in terms of cycling, this is a great,…read morewell-maintained, safe biking path near Boston. Plus, it's super convenient to access - we were coming from Newton and it was a pretty seamless link from the Watertown-Cambridge Greenway. It's about 14 miles from the Alewife T to the Bedford Depot, with plenty of on/off points (I particularly appreciated the 0.5 mile markers). The path hits a number of cool parks and historical landmarks, including Spy Pond, Arlington Great Meadows, Lexington Center, Parker Meadow - you can certainly make a day of biking and checking out each of the stops.
Plus, there are a number of places along the route to stop, take a break, and grab a bite to eat, including Kickstand, Wicked Bagels, Craft food hall, etc.
That said, I would come as early as possible, especially when the weather's nice. The pathway gets congested and whether you're a cyclist or pedestrian, you need to be generally aware of your surroundings. I almost careened into someone when I was trying to pass him on the left and he veered right in front of me. Plus, you definitely have some cyclists who, let's say, seem to be trying out for the Tour de France.
The path is generally smooth with some bumpy, rocky parts, as well as a few points where we had to cross a road and wait for a car to stop and let us pass.