The BWI trail is all paved and goes around the airport. It's not a challenging ride but it's a nice one, and the parking lot for it is right off the runway so you can see planes coming in 50 feet over your head, which is cool. It's a big circle too...so if it's too short you can also go around 2-3 times.
The Baltimore Annapolis trail is also paved. But a very pretty ride. (May not be gully paved. But much of it is. I have't ever ridden it. But I'm driven next to it many times.)
The C&O towpath has many access points, not just downtown DC. It's a hardpack buy not fully paved surface. It's an hour from Annapolis though.
The NCR trail north of Baltimore is similar to the C&O towpath. It's an old rail trail. Crush limestone hardpack. Nothing overly spectacular about it but it gets the job done for a place to ride. It's 45 minutes form Annapolis if the traffic cooperates.
Downtown Annapolis is fun to ride through. It's not a gravel path by any means. All paved and brick streets. But it's a pretty cool quaint downtown area that's a solid mix of Revolutionary and modern times.