The airport is about a 20-minute cab ride from downtown Baltimore, depending on the traffic.
Don’t waste your time with the aquarium. In my opinion, it’s overpriced and overrated. Wait until you can get to a good one, like Monterey Bay or (in my case, on visits home to Australia) the Sydney Aquarium.
You’ve almost got it.
The Metro Bus will take you to Greenbelt - which is the last stop on the Green line in Maryland. From there, you can take the Metro subway to L’Enfant Plaza, then transfer to either the Orange Line or the Blue Line, depending on where you’re going. If you’re going to just Rosslyn (I suspect you are, since that’s where most business people stay) you can take either line, but if you’re going further into Arlington, you’ll need to know which line it is.
You’ll also need to buy a Metro fare card to ride the subway. There are machines there for that. Just do yourself a favor and put $5 on initially - it’ll cost you about $3.50 to get from Greenbelt to Rosslyn (more if you are going further into Arlington).
Well, the Metro Bus that runs there is often called the airport shuttle. The Super Shuttle runs out of there will run you about $30 to get to Arlington. There are also cabs and such. I still vote MARC if you get in early enough. It’ll make stops there at 5:08, 5:41, 6:39, 7:38 and 9:46 in the evening and will drop you straight into Union Station in DC. You can take the shuttle from BWI to the MARC station (they run about every 10-15 minutes), then buy a ticket at the station to DC for about $6.
Not too far at all. About 15 miles or so?
Not “must see” enough to go all the way from Arlington to Baltimore. You would have to take the metro from Arlington to Union Station, then take a MARC from Union Station to Baltimore Penn, then take a cab down to the inner harbor. It would be a good 2 - 2.5 hour trip, depending on how well you timed it. Plus, the time to get back.
Re: The Aquarium – I assume you’ve been to the one in Chattanooga? That’s a great aquarium!
$5 after 5:00 Fridays. That’s the only way to do that aquarium, IMHO.
Make sure you stick to your “reasonable time” window. The DC Metro is not a 24-hour service, and the buses switch over to very infrequent intervals starting pretty soon after evening rush hour. That said, BWI is not quite 30 minutes north of DC’s northerly extreme, and you’re going to the far side of the city – what might be an hour-long trip by car could turn into a 2- or even 3-hour ordeal during rush hour, unless you’re on rails or underground. Rush hour starts around 6:30am and lasts through 9:00am, peaking around 7:30 or 8:00; evening rush runs from 4:00ish through 6:30ish or so, peaking at 5pm.
Don’t forget to check out how close your hotel is to a Metro stop; you could do 90% of your trip underground, only to discover that it’s a 20-minute hike (including luggage) to your hotel from the nearest Metro. Most stops are heavily-patrolled by cabs, so you shouldn’t have a problem getting to your hotel regardless.
You may discover that DC/Baltimore people tend to think about traveling locally like Patton thought of traveling through North Africa. This is a normal and well-adjusted view of the local traffic conditions.
True. But then it’s so packed with screaming children and huge families that it’s hardly worth the effort.
Personally, I like the aquarium, though I don’t think I’d go all the way to Baltimore just to see it. And, yes, you can get there by train. In fact, there are two. Amtrak’s northeast corridor has pretty good service, though you’ll have to cab from the Baltimore end down to the Inner Harbor. Or a commuter train called MARC goes right downtown, but note that it has a fairly restricted schedule. A compromise would be to use MARC to get there and Amtrak to return.
I’m arriving in Baltimore tomorrow night (wish me luck in not making a wrong turn and winding up in Amish country, though if I do I’m bringing back some apple butter and dried corn).
If any D.C./Baltimore/western VA Dopers would like to have dinner or drinks, please let me know. Friday night would be a great time for me.
As they say in D.C.,
uh…
Well, I guess they say “Bye, see you later”.
J
Where will you be around that date?
And if you do, be sure to holler so us Pennsylvanians can say hello. 
Robin
Oh, believe me, if you end up in Amish country I know a couple of guys who’ll give you a very warm welcome… 
Actually, Laurel doesn’t have a Metro subway station.
Wait, did you say this friday night? To bad for me.
As they say in Baltimore, “Buh-bye hon!”
I don’t know if the OP is still looking for responses, but I just made a trip over the holiday weekend – I took Amtrak from Union Station in Washington to B.W.I. – it cost about $12 each way and was very very convenient.
Sampiro, are you still interested in a get-together with local Dopers?
Bump
Wow, that’s nitpicky.