Hạ Long Bay
Hạ Long Bay is the most popular excursion from Hanoi. The Bay is famous for its limestone rock formations which make for fantastic scenery. Pictures and video don't do the area justice. I'd watched several YouTube videos on Hạ Long Bay before the trip and was a bit doubtful that the place would live up to its reputation as the videos did not seem that impressive.
A few tips if you are going: It's worth it to spring for a better boat. Some of the tour boats in the harbor looked pretty bare bones and wouldn't be that nice of an experience. There are many overnight and multi-day cruises. I felt a day cruise was enough and it gave me the full experience of the bay. If you've got time to spare, an overnight cruise is probably a way to relax but if you are pressed for time, it's not necessary. The day trip from Hanoi makes for a long day but it's the most time-efficient way to see Hạ Long Bay.