Battlefield Tour. It will take a full day, at least, but it is well worth the time. The serenity of the park gives no indication of the carnage that took place there.
The park is well maintained, and important sites are well-marked. The monuments are very moving
There are historic sites within the town but I've only had time to see the battlefield. It takes at least two hours but probably allowing four or five would help you be more thorough.