Sapa (Sa Pa) is a small (but very popular) mountain town in the very north of Vietnam. The surroundings are stunningly beautiful and deserve at least a few days to be explored (by scooter and hiking). The weather is unpredictable and it can get very cold (including snow!) so be prepared.
Probably the main reason most tourists visit Sapa is for the multiple ethnic minority groups in the area. You can do hikes guides by some of the groups or just visit their villages. The groups have very distinctive traditional clothing and are mostly very friendly. They will try to get you to buy clothing and jewellery but they didn't seem to ever be forceful.
I got the overnight train from Hanoi to Lao Cai and then hired a scooter so that I could drive the short journey to Sapa. I loved the drive but the cost of the bike was significantly more than you can get in Sapa. You can easily get a bus from Lao Cai to Sapa or just skip the train and get a bus directly from Sapa (I just don't like the buses).