Planning a battlefield trip...driving from IL. Planning, potentially, Gettysburg, Antietam, and Bull Run. Will a day at each cover sufficiently? We have a week.
Yes, one day would be a sufficient amount of time at each battlefield. Gettysburg could use even more time depending on just how much you want to see. Lots of small attractions like the Lincoln Railroad Station, Jenny Wade House, and Eisenhower Farm, just to name a few. I’d recommend at least spending a night in Gettysburg. You could have dinner at one of the restaurants in town or take one of the ghost tours if you are into that sort of thing.
You may also want to find time for Harpers Ferry, site of the pre Civil War John Brown’s Raid. It’s a scenic town on the Shenandoah and Potomac rivers with some great hiking and white water rafting opportunities. Both the Appalachian Trail and the C&O Canal Towpath pass through Harpers Ferry, and you can hike up to the cliffs of Maryland Heights if you are up for a bigger challenge.
While at Gettysburg, make sure you do the Cyclorama at the Visitor Center. And I would also recommend the tour of the battlefield with the licensed guide. The guide drives your vehicle around the battlefield providing you with information, anecdotes, background, strategy, etc. There is a bus tour, but if your budget can accommodate it, the tour with your own guide is well worth it. Our guide happened to have lived in Houston for a while, and incorporated several Texas related places on the tour. Having our own guide gave us lots of flexibility--if we wanted to spend a bit more time at a stop we could do that; he had diagrams and maps to help us understand the logistics of the battle, he answered all the questions we had, etc, etc, etc. You won't get that on the bus tour.
We also toured the Eisenhower home adjacent to the battlefield. It was pretty interesting to tour the only home ever personally owned by Eisenhower and see how they lived--Mamie had the same tile in the bathroom that my mother-in-law had in her bathroom :-)