Greenbelt has a nice route that goes from College Park up to Baltimore's Inner Harbor (on the Chesapeake Bay) which is, from what I hear, a very nice route. So if he's intent on visiting "The Chesapeake Bay" and isn't particular about which part of the bay that might be a good route to follow. If, however, he wishes to visit another part of the bay then it's a different ballpark as you mention (PG county, etc).
Anyway, here's Greenbelt's College Park to Inner Harbor route:
https://maps.google.com/maps?saddr=9...&lci=bike&z=12
and he may have a ridewithgps or mapmyride route as well, hopefully he'll see this and chime in. It's a ~36 mile ride, so would be a bit longer from inside DC but the Metro options could shorten the ride either way and there are MARC trains if he runs in to an issue on the bike that would get him back to DC safely.
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