Last week I tried out Lake Roland in Robert E. Lee Park on the north side of Baltimore City. Boat access was great. There was a good amount of parking and you can put in about 20 yards from the lot. There was a park official there who was friendly and helpful. It's not a very large lake, but it's enough room for a quick relaxing paddle. You do go under a low bridge for the light rail which had some abandonded wasp nests - may want to watch out for this when the weather gets warmer. There is a path around the lake where people were walking, jogging, and fishing. The park is dog friendly as well. There is an area of the park sectioned off for a members only dog park, but the trails are open for anyone. I saw some fish spawning in the flats and a few birds of prey soaring overhead. There was a decent feel of wilderness considering how close it is to the city. I was only there for about an hour, but I didn't see any other canoes or kayaks.
Yesterday I tried out a stretch of the Patapsco river near Ellicott City. The access was off of Daniels Rd near I-70. Again, great boat access. I managed to get one of the few parking spots right by the water, but people were parking along the road when those filled up. The put in point is about 15 yards from the parking area. The river was very scenic with plenty of wildlife. The water was calm and clear. There is a path next to the river that had mountain bikers, walkers, joggers, and even horseback riders. This place was also dog friendly. The current wasn't very strong and I went about 2 miles up river. About a mile and a half in, there was a stretch where the water was no more than 6 inches deep. I got through it fine, but if the water level gets low this might be as far as you're gonna get. Towards end of the 2 miles there were a couple small rapids (class I/II at most), but the water was still for the most part. I did see one other kayaker and there was a couple dropping in a tandem canoe when I was leaving.
Both places were quite enjoyable and easy to get to. Both were free (no permits needed) and boat friendly. Overall, very good places to do some flatwater paddling close to the city.
Links to Google maps for both places below.