Sapa, set in the mountains of the far north of Vietnam, is the perfect base for enjoying some amazing mountain scenery and experiencing the culture of the Vietnamese ethnic minority hill tribes.
On the far eastern end of the Himalayas at 1500m, the town of Sapa was originally built by the French, looking for a site to build a hill station to escape the searing summer heat of Hanoi. It has an alpine feel, but with a twist - everywhere in the town you'll see the hilltribe women in traditional dress who are very good at persuading you to buy trinkets and textiles, they're great fun to bargain with.
The town has its charms, but the real joy is the surrounding stunning scenery. There are plenty of trekking and mountain biking opportunities for the energetic, but you can still experience the lifestyles of the Hmong and Dao people in other ways if you're not so keen on strenuous exercise.
There are many trekking options available from Sapa, from a short half-day trip into a local Hmong village for lunch with a family, to a full trek to the top of Mount Fansipan, the highest peak in Vietnam (2 or 3 days with overnight camps). There are also excellent guided mountain bike trips.
As well as organised adventures, there is much to be said for just getting on a motorbike and riding off into the surrounding countryside and villages, or putting on your hiking boots and heading off to see what you can see.
If you have the time, you might want to consider the Ha Giang area as a side-trip, or even as an alternative to Sapa - not easy to reach, and limited accommodation options, but much quieter/less touristy than Sapa, and truly beautiful scenery.