Dan - Great you are coming to Bmore! You will love the B&O Museum!
Yes G&M crab cakes are tops for sure! You might also check out Fells Point which is the oldest part of the Baltimore harbor ( cobble stone streets etc ). There is a seafood restaurant in Fells Point named Berthas which has great muscles! Fells Point has a charm all its own, however, parking your car is not so easy. Fells point streets still have some of the old railroad tracks with tight curves where Docksiders were used at one time.
There are many nice restaurants on N. Charles Street just south and north of Mt. Vernon Circle ( which hosts Baltimore's Washington Monument ). Lots of beautiful architecture in this area. While you are on Charles you can visit Penn Station, Amtraks own. A few blocks away, in the Mt. Royal neighborhood why not visit Mt. Royal Station of B&O fame. This station is now part of the Maryland Institute of Art, however, the outside architecture is as beautiful as the day it opened. The train sheds are still there as this is the entrance to the famous Howard Street Tunnel built by the B&O and now owned by CSX.
Also take a ride on the Baltimore Light Rail. You can ride it from end to end if you wish and see lots of cool stuff. Part of it traverses the old Northern Central Railroad line which ran from Baltimore to Harrisburg.
If you want to check out the Thomas Viaduct you may do so. It's located in Elkridge, Md. just off Rt. 1on Patapsco Ave. ( about 20 - 25 minutes outside Balto give or take a few traffic lights ) which you can pick up in Baltimore, somewhat close to the inner harbor. This bridge, built by the B&O in the 1830's?, is still in use and carries the CSX - MARC Camden line across the Patapsco River.
If you are a concert goer check out the Baltimore Symphony at the Meyerhoff Concert Hall located in the Mt. Royal neighborhood. Great acoustics! Other artists ( not just classical ) play there as well. There is also the Rams Head Live which brings in national and local artists.
Ellicott City is about 35 - 45 minutes away, depending on traffic. It has the first terminus of the B&O. The old stone station still stands and is part of the B&O museum, however, not for long .... Howard County Parks is taking it over from the museum soon.... so check with the B&O museum to see how long you ticket is good.
Oh yes - by all mean check out Camden Station which is now a sports museum and part of the Orioles Camden Yards complex. Camden Station is still in use by Baltimore Light Rail and MARC. Of course the interior of the actual station is not used for railroad purposes. All ticket transactions etc. are out of a baggage car and automatic machines.
As for train stores: MB Klien in Cockeysville ( about 25 minutes from downtown ) is really the only game in town ( or in this case out of town ). There is not much of a retail show room any longer as they are using what used to be retail space for their surging internet business: modeltrainstuff.com At least they seem to be doing well financially. There is a small retail area within the store.
I hope this helps in some way.
Do enjoy your stay!