The world famous Ha Long Bay is worth the hype.
Accomodation
Halong Fancy Hostel: another standard Vietnam Hostel. Good food, cheap beer, and friendly staff who know the area so can help you organise trips to the bay.
Activities
The Bay: I got a day boat tour through the bay, stopping at various islands to go through caves, swimming, and then kayaking in a lagoon within one of the mountains. Also gave us loads of (sea)food that was of course delicious.
Quang Ninh Museum: a local history museum, it's very big so may take some time, but is certainly interesting with many examples of real equipment/clothes used in the past.
Poem Mountain (Bai Tho): the best view of the bay. To get onto the mountain you have to pay to go through somebody's house, which backs onto an old trail, where you then climb up a small cliff to reach a path. After that it's just stairs through some overgrown jungle, but it's worth it. It's not an official business and I'd recommend a bit of fitness, but it's a brilliant view. If you go for the sunset like I did, take a light. The stairs are hard to navigate in the dark.
@rkmkiosk It was pretty damn warm. It's supposedly cooler the further north you go but I experienced very similar temperatures throughout the entire trip with just a couple of outliers. It was mid to high 30s (celsius) pretty consistently. It's very very high humidity as well, so google was telling me it 'felt like' closer to 50 some days in Hanoi. That was particularly bad though, Id adjusted to 40ish after a couple of weeks.
The only exceptions were Dalat (a mountain city, reached as low as 18 at night, and Ha Giang near the Chinese border would rain a lot so it was closer to 20s than 30s. Still overall very warm though, I wore the same clothes just with a waterproof on, and the rain wasnt too bad because I was so hot!
How hot is it in june? Btw amazing itinerary