If you do rail-trails, the Northern Central is an excellent one. It can be very crowded at the southern end, so it's worthwhile if you can drive to one of the parking areas further up. I usually go to either Monkton or all the way up to Freeland and ride across the border into PA.
The only other riding I've done in the B'more area is Tour du Port, which went through a great variety of neighborhoods last year, but I don't know if you want to be on the city roads during weekday traffic.