We, hubby, myself and 13 yr old son, will be visiting Harper's ferry, Antietam and Gettysburg in August. We've thought of making a day trip to Maryland if we can find something interesting do to that isn't too far of a drive from Gettysburg. Any suggestions? After two days at Harper's Ferry and two and a half days at Gettysburg, we'll have an extra day left and wanted some advice. Thanks!

Don't know what kind of things you are interested in, but in PA you could spend go to Hershey, or Fallingwater or Johnstown to see the Flood Musesums, etc. All are doable in that time period.

Come to the Inner Harbor in Baltimore. You could spend a week there.

Thanks for the replies, we've heard only good things about Hershey park, but decided against going on this trip.

The Inner Harbor in Baltimore sounds great---can you give me some more info? I'll also so a web search.

Baltimore's Inner Harbor is certainly worth a visit and has lots of things that you and a 13 yr old will enjoy such as historic ships including the tall ship Constellation and a submarine, fantastic aquarium, street entertainment, Nearby Fort McHenry, Little Italy, etc. However it is about 60 miles or an hour and a half drive or so from Gettysburg. You would want to put aside an entire day at least. Get an all day Water Taxi pass that will take you to most of the sights. If you stay overnight, I would recommend Henderson's Wharf Inn in Fell's Point.
Two whole days in Harper's Ferry seems like a lot unless you intend to spend time white water rafting. In Harper's Ferry you might also enjoy the Ghost Tour at night if they are doing it when you are there.

Forgot to mention, IMO the best way to tour the Gettysburg battlefield is by hiring a licensed guide at the visitor's center for about $35. The guide will ride with you in your car and they really know the interesting details that you might not learn elsewhere. To hire one you need to get to the visitor's center early because they are quite popular. A good less expensive alternative is to rent a self guiding tape.