We went in April. We only had 2 half days in HCMC, up front and at the end.
The first half day we went to war remnants museum (taxi from the Majestic). A must do! Then cyclo to the Reunification Palace - worth going to - then creme caramel and a beer near Notre Dame looking out onto the park. Followed by a little shopping, swim at the hotel, lunch, pack and over to the airport for 4.30 flight to Hanoi.
Our second half was after we arrived at the airport at 2.30 from Hoi An, taxi into Saigon, walking, shopping and a really neat dinner at a "Tapas" bar (french/vietnamese food) and a few glasses each of wine from South Africa, South America, France, New Zealand, Oz, Italy Spain etc (can't remember which we DIDN'T have!). Taxi to airport at 7.00 for flight back to Oz.

Ahhhh, memories! You will love it!

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Smile and the world you are travelling in will smile back at you