Terrific. Just rode through Belmont on my way home yesterday after a long ride.
No scenic route between Belmont Center and the Charles that I'm aware of. For commuting purposes, I find most of the roads around there pretty good: Belmont, Brattle, Mt. Auburn. They do get nasty around Elliot Bridge. Arsenal is doable, but not plesant (I love that it has "Share the Road" signs and a bike lane somewhere in the middle of it that doesn't really connect to anything at either end
).
Your 2 options are to 1) work your way straight south to the Charles or 2) work your way over to Harvard Cambridge and pick up the Charles there.
Once you find your route, you'll find things going much faster. I still miss riding into Downtown on the Esplanade in the Spring. Enjoy.