If you're aiming to hit Fenway and the Gardner or the MFA, it would probably be better to take the Blue Line. I don't know that you can buy a day pass on the Silver Line bus, and if you can't buy a pass or a stored value "Charlie Card" onboard, then you will be forced to pay a higher fare per ride. (The single-ride fare structure just got more complicated and more expensive as of Jan. 1.) The Blue Line is gross, but you can definitely buy a day pass or a Charlie Card at the Airport station on the Blue Line. From there, it's only a few stops to a direct connection with the Green Line at Government Center station. Fenway and the museums are on different branches of the Green Line.