Antietam National Battlefield commemorates the site of the bloodiest single day in US Civil War history. The battle of Antietam or (Sharpsburg)was important in many ways. Robert E.Lee made his first attempt to invade northern territory and was narrowly defeated by a superior Union force despite heavy causalities. After the battle the Union General George McClellan was relieved of overall command because of his failure to pursue Lee with his superior force.
The most important consequence of the battle was that Lincoln was able to use the victory to announce the Emancipation Proclamation. Although it did not actually free any slaves at the time the symbol was undeniable.
The battlefield itself is excellently presented with a good visitor center and a nice driving tour winding its way through the key areas of the battlefield. The park also has some wonderful walking opportunities.
The Burnside Bridge was the key site of the battle. Park above the bridge for a great overview of the area and then take the short walk to the bridge.
During summer weekends you may catch some re-enactors.