Parking stinks over by the Park Plaza. There is one garage (the one by the Charles Street Playhouse attatched to the hotel there) that is relatively inexpensive ($18)if you stay there less than 12 hours, but then you'd have to move your car to a metered space. If you don't mind playing musical cars, that could work. Then there's the option of parking at Alewife (it's one of the few T stops that you can park overnight in) and taking the T in. Of course you may have trouble if there is considerable luggage to drag around.