There was a post on this a while back--some hotels in Baltimore allow you to stay there either before your trip or after your trip and leave your car parked at the hotel. My parents did this, but they can't remember the name of the hotel.
We stayed at the Suisse Chalet last February and it was about $95 for the night. It was supposed to include 7 days of free parking and $7 a day for each additional day you needed. We stayed for 9 days and they told us not to worry about the extra 2. The rooms were small and I had to sleep on the floor because there were five of us. This November, we are trying Amerisuites. It is right around the corner from the other one. The price is about $120, but you get half off with an Entertainment Book coupon. I think you have to book at least 21 days in advance. Go to www.amerisuites.com and check it out. I will probably end up calling them to make sure about the coupon and the parking, but my friend stayed there in January and loved it! Good luck.