We've just returned from a month in SE Asia. 1 week in Cambodia, 1 week in Myanmar and two weeks in Vietnam.
We travelled from Hanoi to Saigon during our trip, taking in a few sights along the way. It looked a little something like this;
2 days in Hanoi - plenty of walking tours available. See the French Quarter and Old Quarter. We stayed in a really nice (and relatively cheap) hotel called Arts Trendy - very helpful staff who booked us train tickets and tours, as well as being in the heart of Old Town. You can also plan on visiting the train line that runs through the narrow streets of the city. Aim to be there by around 4pm - there should be a train before 5pm, then around 6.30pm (ish...)
Overnight sleeper train to SaPa in the North - border with China. A really, really stunning mountain town surrounded by terraced rice fields and overlooking the Fansipan Mountain. You can do plenty of walking around this area, many of the local tribes have started up homestays and do tour guide duties. You can also visit about 3 large, local waterfalls, take the cable car to the summit of Fansipan mountain (it's normally in cloud though) or take a day tour to Bac Ha Market (on Sunday's only) which is famous for the colourful Flower Hmong Tribe, selling clothes, animals (puppies, kittens, pigs, ducks, ox, etc).
From Hanoi you can either do a day trip to Halong Bay, which we did, and regretted. It's a 4.5hr bus journey out, 2-3hrs on a boat, then 4.5 hr bus journey back. We didn't enjoy Halong at all, probably for this reason, but also because it's so crowded and incredibly touristy. Boats jostling for positing everywhere. There are plenty of tours that will do one or two nights on the boat so you get to see more of the bay, but I'm not sure that would've been any better.
We did do a day tour to Tam Coc, dubbed as the inland Halong Bay, which we loved. We took a private car in the end as we had to be back early for our train, but were there by 8am, one of the first tourists out on the river with a local lady rowing us through the towering limestone karsts, through low tunnels and among the rice fields. It was much more peaceful and serene, and you can walk up to a temple and overlook part of the river too.
We then headed down to Dong Hoi, caught a taxi to Phong Nha and visited the Phong Nha Kebang National Park. This place is famous for caves. there are plenty you can visit on day trips, either rowing down the river or a short guided walk in to the jungle, but we went all out and took a 1-night, 2-day trek/camp to Hang En, the third largest cave in the world. This is the cave where you have the well known photos of the tents inside the cave entrance with the ray of sun light coming in - that sun usually only happens around March (FYI). It wasn't cheap, about $300 each, but was definitely my most memorable experience. The walk is about 11km each way. Out to the cave is down the mountain (relatively easy) but you have about 2km back up the mountain on the way back, and it's steep!!! The other option for Phong Nha is Son Doong, the largest cave. You won't get on to a tour now for 2017 as they sell out in about 24hrs, but you can sign up to book a tour in 2018. It's about $1,500-1,800 and takes 5 days, you have to pass a fitness test and looks incredible. I'm planning to go back in 2018 just for this.
From Phong Nha we caught another train on to Hoi An. We ended up having four nights here - it's very peaceful, beautiful old town, and in the centre, they don't allow cars or motor bikes (for most of the day) which makes it nice to wonder around. We did a day trip to Cham Islands (
http://www.vietnamscubadiving.com) and did some diving (not brilliant and a lot of the waters had problems with jelly fish infestations). We had considered camping on the island as they also offer that, or staying in one of the homestays. We also did a half-day cycling tour out of the town to Coconut Island, my wife rode a buffalo and we ate some local shrimp salad and Vietnamese Pancakes. (
http://hoianecogreentour.com)
We were in Hoi An for the lantern festival and regretted it. Suddenly Hoi An turned from a peaceful, pretty town in to a horrible commercial centre packed with hawkers, touts, so many tourists, you couldn't move. It spoilt the place for us slights, but each to their own.
We took a flight from Danang (closes airport/train station to Hoi An) to Saigon and spent a day doing the War Remnants Museum in the morning (it closes at 12:00) and the Chu Chi Tunnels in the afternoon. Our hotel book the tunnels tour as it's about 1.5hr drive out of the city.
Our last day we booked a private tour to the Mekong Delta. We wish we had a bit more time (we were greedy in Hoi An) and did an overnight tour but although it was pricey, we really enjoyed the off-the-tourist-trail nature of our tour (
http://waterbuffalotours.com/Tour/Detail?id=1). We visited a local village market, which obviously has very few white people visiting as they wanted to take our photos as we wanted to take theirs. We had lunch on the ocean front - delicious fresh seafood - then took a boat tour along part of the Delta and through a coconut plantation.
Emirates now fly in to Hanoi (through Yangon, Myanmar) and Saigon (both through Dubai).
If you want to ask any specific questions, feel free to drone me a PM.
Sorry for the long post.