This is a short loop for riders that live in Cambridge or Somerville. Get to Powder House Square and ride out towards the Mystic River Parkway. Get on the parkway at the Alewife Brook Parkway roundabout and follow the river out all the way to the Lakes. Keep going on the River Parkway and you will hit South Border Rd of the Fells, which takes you around the edge of the Fells to the roundabout over 93. Follow signs to the Fellsway and you will ride through the Fells. Then, turn right where Stoneham starts and that will take you around Spot Pond, all the way back to the Fellsway. From there retrace your way back.
Once you get out to the Mystic River Parkway, the route follows the river which includes lots of nature and trails near the road. The Mystic Lakes themselves are picturesque on a nice day, its about as much nature as you can get this close in the city. After that you will hit some smaller town before the Fells Border Rd. This road has the forest to your left and residential houses and streets to your right. The Fellsway takes you out to Spot Pond which is also a nice nature area.
You obviously have to slog through the city first to get to the Mystic Valley, and even then there will be some traffic you encounter if you go at the wrong time. The roads themselves are quite decent for Boston standards, they recently redid a large portion of the Border Rd so that helps. Again its not much but its the quickest way to some twisty roads without going too far, great for a quick ride on a weeknight when you just need to get out.
The Mystic Lakes and Fells/Spot Pond are nice nature areas. Otherwise there is not much on the way, you do pass by Stone Zoo on the way around Spot Pond.