What to do in Cambodia and Vietnam for 6 days?

In Siem Reap: visit Angkor Wat complex (1-3 days) and some other  surrounding temples.

In Hanoi: Temple of Literature, Ho Chi Minh Mausoleum, One pillar pagoda, Museum of Vietnamese History, Ethnology museum, military museum, Hanoi Old Quarter Hotel, water puppetry, Công ty TNHH Bát Tràng Gốm Tinh Hoa, Vạn Phúc silk village. Also, strongly recommend you try Hanoi street food (I and my friends did it with Indochina Best Travel & extremely love it :) )

In Ho Chi Minh City: Cu Chi tunnel, Church, Ben Thanh market, Reunification palace central post office...; far away is Cao Dai temple. If u have much time, visit Mekong delta with floating market as well

From Siem Reap, you should travel around Ho Chi Minh City, you can homestay at local people in Mekong Delta and join floating market in the early morning. Personally, you should book or refer tour via agency like Mekong Delta Tours 

In Phnom Penh Phnom Penh, you can visit

Choeung Ek Genocidal Center, centre market, Wat Phnom, Bến Thành Market, Notre Dame Cathedral, Central Post Office. 

In Hanoi

Hoan Kiem Lake, Hanoi Old Quarter, Đường Lâm ancient village; especially there are quite a few delicious food in Old Quarter.

Hi Edmund,

For Siem Reap sightseeing you can do 1 day temple tour for if you have short time here. I can suggest you Angkor Wat Small tour, all the main temples are in this tour such as Angkor Wat temple, Angkor Thom, Ta Prohm Temple. There are many more to do and see depending how much time you will spend over here.

Thank

Sina

We hired a local guide to take us to a few temples in Siem Reap.  We normally don't hire any guides, but we are so happy we did this time around.  They have so much more in depth knowledge, and can point out little details.  Our favorites were, of course, Angkor Wat-try to go for sun up.  After the sun rises, head over to a different temple because everyone else will just stay and tour Angkor.  You can beat the crowds by heading directly to Ta Prohm Temple and wander around quietly.  Then head to the temple where every face is different-Angkor Thom's The Bayon.  After, head back to a much quieter version of Angkor Wat.

We loved wandering around Pub St, taking in the markets, especially the night market.

In HCMC, we really enjoyed the The Independence Palace, Nha hang Ngon (where the owner brough 12 of the best street food vendors under one roof), cocktails at the historic Hotel Caravelle on the rooftop bar, and wandering the many stalls (and the craziness of the streets just outside) of the Ben Thanh Market.

In Hanoi, it is worth it to visit the Ho Chi Minh Museum-take in the grounds surrounding his palace, the museum and the mausoleum.  We enjoyed wander through the Temple of Literature and wandering around the lake right in the center of town.  Make sure to take in a water puppet show!  You can book a street food tour with a local, along with the tickets to the show-highly recommend.  Lastly, our favorite meal was at KOTO- which stands for "Know One Teach One"-they train under-privileged youth for a career in the hospitality industry.