I probably have a bike I could loan ya, well, I definitely do, just a matter of how much liability you want to take on in such a loan
Would be happy to take you out for a ride also, though I live about 5mi south (red line to ashmont) from where you are staying.
As far as "cool shops to visit" well, not much really, pretty service/sales oriented in the city, not many little hole-in-the-wall museum-style shops with any history unfortunately. Somervelo in Somerville is about as close as you'll get. Beyond that, Ride Studio is a great place to visit on your way in/out of the city on a ride. Great coffee and plenty of eye candy.