Edmund (CET) and I have been to Gettysburg several times. The best way, in our opinion, is to go by car and arrange to be there for two to three days. The first day, see the museum at the visitors' centre, especially the "electric map" presentation, and sign up for a private tour with a licensed battlefield guide, which is what you do the second day. Definitely do the self-guided tour by car or on foot, which can take you an hour (by car) or all day, depending on how energetic you are and how much you think you can learn from walking the ground.

You should also take in the special presentations by the park rangers. These are usually given within easy walking distance of the visitors' centre, which is less than 100 metres from the "high-water mark of the Confederacy", the spot where Pickett's Charge fizzled out. If you can manage a horse or a mule (stronger than horses but more wilful), you might consider taking the horseback tour, which covers parts of the site that you aren't allowed to drive to. I believe that bicycles are also allowed.

The licensed battlefield guides are the Gettysburg equivalent of an old-fashioned London taxi driver, but military. They know every bump and tree of the battlefield, and drive you around in your car telling you exactly what happened and when at every spot (and why it mattered), and answering any and all questions you can possibly dream up. Some are ex-soldiers who have retired into the job of their dreams, and all are worth the price of the tour. If you go on a bus tour, the commentary will be given by one of these guides.

Don't for God's sake go during the first week of July, the anniversary of the battle, when the place is crawling with re-enactors. A week day in spring or fall is usually good, although in early summer you could run into large school tours.

Gettysburg is the most developed battlefield site I have seen. It is closely followed by Appomatox Court House, an entire village that has been preserved as an historic site. Antietam, too, is well worth a visit, and you could easily spend a week in the Fredericksburg-Chancellorsville area.

Just beware of the Martha Stewart-esque bed and breakfast establishments, places so embellished that you have a hard time finding a spot to sit down. All the ones we've been to offered a terribly fancy breakfast with dreadful coffee.