Vietnam's Demilitarized Zone, or DMZ, is the area around the former border between North and South Vietnam. Historically it was a narrow band of terrain extending from the Laos border to the coast, five km on either side of the Ben Hai River, roughly on the 17th parallel north latitude.
The area saw heavy fighting in the war, and ruins of old American military bases still exist. Even if you're not interested in the history, the area has some spectacular mountain scenery and rugged jungles.
While the actual border was marked by the Ben Hai river, most historical sights (i.e., American bases) are along Highway 9, which runs parallel to the river several km to the south. This road runs to the Lao border and continues onward.
The area's only major city is Dong Ha, on the coast. It's on Highway 1, and easily accessible from Hue and Da Nang. Lots of travel agents in Hue offer convenient day trips.
Unless you have your own transportation, you'll need to hire a taxi or take a guided tour to see the sights.
Ben Hai Bridge crosses the river and marks the former border. There is a monument on the north side.
Truong Son National Cemetery is Vietnam's national war cemetery.
Camp Carroll is located south of the highway near Cam Lo village. A small monument features a caricatured depiction of a defeated American soldier.
The Rockpile was Marine outpost built on top of a huge outcropping. Though it's inaccessible, it's a prominent sight from the highway.
Dak Rong Bridge is midway along Highway 9, and is the starting point of Highway 15, which leads south to the A Shau valley and the infamous "Hamburger Hill". Though not entirely legitimate, there is a monument commemorating it as a point on the Ho Chi Minh Trail, (called Đường Trường Sơn in Vietnam).
Khe Sanh was the site of a U.S. base which fell under attack in 1968. The old airfield of red dirt still remains.
Some local tour operators such as Active Travel Vietnam, Voyage Vietnam (in Hanoi) offer an amazing trip along Ho Chi Minh Trail by motorcycle .
Though you'll probably encounter vendors selling GI dogtags, lighters, and other paraphernalia, you can be sure that none of them are genuinely from the war.