I would suggest to leave it unplanned and explore further something that you found particularly interesting the first couple days. A good guidebook is a must, and ideally ahead of time, and to bring along. The best one is
this (and has also a lot of interesting information about things that they are not obvious today, like weapons & range, what is a line vs. a column, what distances people traveled when etc.), but unless you are driving or planning to check baggage, is not recommended.
This one is a good compromise. The first time I went to the battlefield (and it happened just because I was in the area, and I was pretty much ignorant about the Civil War) something in particular struck me, because I did not expected it: The incredible number of monuments there. There is a nice guide book for
that, if you are interested in the subject. A good map of the battlefield (like one that folds and unfolds) is a must even for your tours.
As far as the museum goes, the bookstore is a must, you can find everything I indicated above (and much more.) The museum itself is ok and it is more of a generalized view of the civil war and the battle with little vignettes on individuals. The cyclorama is ok if one is interested to get a glimpse of what early tourism in Gettysburg was all about. Bit off the beaten path, but well worth it, is the
Seminary Ridge Musuem. Same building, same cupola that Buford climbed (and you can tour the cupola and have a look around...) The "museums" in town are tourist traps for sure...