The Cao Dai Holy See: The spiritual home base of the Cao Dai religion, a unique contemporary faith, the Holy See is a fantasyland of colored mosaics and elaborate painting. Followers are dressed in white turbans during the picturesque daily procession.
Tomb of Khai Dinh: The egotistical, eccentric emperor Khai Dinh left behind a tomb that is a gaudy mix of Gothic, baroque, and classical Chinese architecture. Quite unique.
The Tunnels of Vinh Moc and Cu Chi: Faced with devastating air raids, both of these sites supported large groups of soldiers and civilians who used the tunnels as supply lines, escape routes, and as bases for waging a devastating guerilla campaign against U.S. forces. Day trips to either site are memorable.
Hoa Lo Prison (aka Hanoi Hilton): Home to U.S. pilots -- including John McCain -- who were shot down during the Vietnamese war with the U.S., Hoa Lo Prison is now a small museum (most of it was demolished for a high-rise). A good glimpse into Vietnam's grisly past.
Reunification Palace: In 1975 tanks rolled over the gates of the prime minister's palace, signaling an end to the Vietnam War. You can see the actual tanks on-site.