The majestic temples of Angkor Wat are just a short flight from Ho Chi Minh City or Hanoi, and many travelers extend their trip to visit this archaeological wonder.
Day 1: Ho Chi Minh City/Hanoi to Siem Reap
Fly from Hanoi or Ho Chi Minh City to Siem Reap. Get comfortable at your hotel: The Raffles Grand Hotel D'Angkor is a palace, and Hôtel de la Paix in town is a contemporary luxury.
Day 2: Angkor Wat
Set out for the temples before dawn, taking in the morning glow from atop Bakeng Hill. Visit the main temples, including the many sights of Angkor Thom -- including the Bayon, the Baphuon, the Terrace of the Leper King, and the Elephant Terrace -- and then hit Preah Khan (if you have time) before lunch. In the afternoon, take in the jungle temple and everyone's favorite, Ta Prohm. Make a few more stops, maybe at Banteay Kdei and Sras Srang, before sinking your teeth into the big temple, Angkor Wat, where you can spend hours going through the relief carvings like the Churning of the Ocean of Milk. Take a guided tour to get the most out of this experience. Climb the three tiers of the temple and enjoy the sunset from the top.
Head back to Siem Reap to enjoy a fine meal (for local food, try Khmer Kitchen), beers, and backpacker babbling. If it's a Saturday, take in the show by Beat Beatocello at the Kantha Bopha Foundation, where Beat (aka Dr. Richner) entertains an audience with his cello while telling of the foundation's aid work and begging for funds (all in good fun).
Day 3: Angkor Wat
Watch the sunrise from Sras Srang or Angkor Wat, and then hit your favorite temple again or any you missed in the main Angkor compound. Many use Day 3 for trips out to the likes of the Roluos Group or as far as the crumbling jungle temple of Beng Melea. See the sunset from Bakeng Hill. Catch a morning or afternoon flight back to Ho Chi Minh City or Hanoi.
Option: Add an extra day and hop on a flight to the crumbly old colonial Phnom Penh and visit the city's rather grim sights focused on the genocide in Cambodia: Tuol Sleng Museum and the Killing Fields. Explore the city's large Central Market and maze of dusty streets, or enjoy a walk along the riverside Sisowath Quay or a ride across the Japanese Bridge for a night at a local minstrel show.