A good way to get to the Arboretum is to take the linear path that parallels the Orange Line. It starts at Back Bay but you can pick it up behind the Orange Line Mass Ave. station. (If you don't want to ride Mass Ave down from Cambridge you can take sidestreets or become the first person to ever put their bike on one of the CT bus front bike racks--*someone* has to do it.) The path has one lane for cyclists and one for pedestrians, although they keep flipflopping. Occasionally the path turns into sidewalks and you have to find where it picks up again, but it's in good condition and takes you all the way down to Forest Hills. Scenery is old-brick-and-gardens for the first bit and industrial-chic as you go down thru Roxbury and Jamaica Plain. (You can one-up Cambridge Liberals by starting a sentence with "The other day when I was biking thru Roxbury Crossing...") On the way down you pass near Bikes Not Bombs (www.bikesnotbombs.org). Franklin Park, which has a zoo, and the Forest Hills Cemetery, which has installation art, are also down there next to the Arboretum. Not sure you can bike in the cemetery, you can't in Mt. Auburn Cem.
You can go back on the older, multi-use paths that take you thru the Emerald Necklace (park network designed by Olmsted). I seem to remember the path tries really hard to lose you at the Rt. 9 crossing. When you come out at Audubon Circle, if you do not want to try rotary cycling you can walk across...& shop for bike doodads at the new REI in the Landmark Center. The CT2 goes by there and could take you & bike back across the river, although it heads for MIT rather than Harvard.
Enjoy!
Neon