Yardtrain - Enjoy the 611 and Strassburg! I've seen her both on static display and in live action , took the cab tour given by the chief mechanical officer back in 2019 too.
I've lived in the Baltimore/DC area all my life.
You've already been given lots of great suggestions so here are a few more .. LOL! Yes definitely check out the B&O Museum first. Then check out other rail sites/tour that Nickaix suggested. The Thomas Viaduct is a beautiful structure! You may also want to check out the Baltimore Street Car Museum which re - opens Aug. 15 according to their website. The Ellicott City Museum is on the old main line and is the first railroad terminus in the US. The famous race between the horse and Peter Cooper's Tom Thumb took place from Mount Claire ( home of the B&O Museum ) and Ellicott City which spanned 13 miles. The first US president to ever board a train boarded that B&O train at the station in Ellicott City and rode to Mt. Claire Station.
If you are into RR architecture ... the old B&O Mt. Royal Station is a beautiful structure. It's now part of the Maryland Institute College of Art, but the exterior is preserved and looks like it did back when it was first built. Also check out Penn Station ( now serving Amtrak ) built by the Pennsylvania RR. Camden Station is now home to a sports museum and next door to Camden Yards Stadium, home of the Orioles.
The best crab cakes are NOT to be found or tasted at the Inner Harbor. Those crab cakes are found at G&M Resturaunt in Lithicum Heights about 15 minutes outside of Baltimore. Your tastebuds will not be disappointed, I assure you!! G&M is a favorite of locals and is known all over the US as they fly their crab cakes out of BWI airport to customers throughout the country. Beware that due to a shortage of crabmeat and high demand the cost of crabmeat has skyrocketed ... hopefully temporarily.
Out for a night - check out Fells Point, the oldest neighborhood in Baltimore. Located on the Harbor ( not the Inner Harbor ) Fells Point's cobble stone streets offer lots of eclectic shops and restaurants. It's a fun place to be at night! Due to it's popularity parking can be a bit of a challenge.
While checking out the Inner Harbor you can check out the USS Constellation, the last sail ship built for the US Navy. You may need to make reservations ahead of time to visit the National Aquarium located in the Inner Harbor. It's a great Aquarium!! Well worth a visit if you can get tickets. Heck it might even give you a reason to purchase a Lionel aquarium car if you don't already have one... LOL!
You can also take a ride on Baltimore's Light Rail.
Baltimore and surrounding area offers the usual array of chain motels/hotels from Motel 6 to high end luxury Hotels. AAA can probably give you best advise ... if you are a member.
At any rate enjoy your trip!!