Biking the Charles is quite nice... but like I said, not on the weekends... that's when the weekend cyclists, kids, rollerbladers, joggers, stereo headphone junkies, etc literally swamp the path.

Its perfect for an early evening extended ride home though... especially if the clouds are just right and a storm might be brewing -- I used to live on Beacon -- so I would walk (or ride and leave my bike locked under a ped-overpass) and just stand on the bank and watch/feel/let the wall of water roll over me... also ride to castle island in dorchester and the wharf over by the aquarium, charlestown navel yard is cool too.