Baltimore at Christmas

Our daughter will be working a show in Baltimore through the month of December. My wife and I are planning to fly up and spend time with her 12/25-28 arriving mid afternoon on the 25th. They have no performances on Christmas day or on the 26th. I know that we want to make a trip to Ft McHenry on one of our days and we'd like to head into DC as well. We'll be staying downtown near the Inner Harbor. How is public transport (including rail to DC)? Any suggestions for must sees?