http://www.aacounty.org/RecParks/parks/trails/bandatrailpark.cfm [1]
The Baltimore-Annapolis Trail technically starts in the town of Glen Burnie, so plan to begin your ride at BWI Airport, and take the BWI Trail about 12 miles to the northern end of the 13-mile long Baltimore-Annapolis Trail (the URL included in this listing will get you to maps of both trails).
The ride crosses only a few roads, and mostly takes you through woods and behind lots of residential areas. Built on a former railroad line, this is a flat trail that challenges riders with distance, not elevation gain. There are plenty of places to stop for food and drink along the way, including (during the summers) a great snowball stand at the halfway point of the B-A Trail.
Links:
[1] http://www.aacounty.org/RecParks/parks/trails/bandatrailpark.cfm