Sure, I'd be happy to assist! Visiting Gettysburg, Antietam, and Manassas offers a captivating glance into U.S. history, mainly revolving around Civil War battlefields. Here are some traveling tips that you would need:

Best Time to Visit:
The ideal time to visit these civil war sites is during the spring (April-June) and fall (September-October). The weather is mostly pleasant, making it perfect for outdoor activities.

Route and Accessibility:
These sites are all located within a few hours' drive from Washington D.C or Philadelphia, making them easily accessible via road. Gettysburg is about a 90-minute drive from both Philadelphia and Washington, D.C. Antietam is about an hour from D.C., and Manassas is a half-hour.

Altitude and Acclimatization:
All three sites are low altitude areas, and acclimatization will not be an issue.

Accommodation and Booking:
There are plenty of hotels, motels, and B&Bs in these areas, and you may want to book your accommodation in advance. Accommodations in Gettysburg can be especially busy around the anniversary of the Gettysburg Battle in early July.

Clothing and Essentials:
Pack comfortable clothing suitable for the weather during your visit. Don't forget to bring comfortable shoes, sunscreen, hats, and water bottles for your trips to the battlefields.

Health and Safety:
There's no special safety issue in these areas. But stay hydrated and take care of your personal belongings.

Local Etiquette and Respect:
These sites are all national historic sites, so treating them with respect is crucial. Stay on marked trails, and don't pick up or take any objects, such as bullets or other artifacts, that you may find on the ground. Doing this is not only respectful, but it is also the law.