Most enjoyable part of Vietnam: Trekking around the Sapa Valley with an awesome locally hired tour guide.
Sapa and the surrounding valley offer amazing views of the mountainous terrain. The town of Sapa has quickly become a tourist magnet as locals from the surrounding villages still walk around in their traditional clothing. The eerie feeling and cool temperatures caused by the fog along with endless hills of rice patties and hundreds of hiking trails lined with basic villages makes this a welcome escape from the cities of Hanoi and the beaches of southern Vietnam.
As an outdoor enthusiast, having lived in the American Pacific Northwest for the past 4 years, I can tell you the backpacking around this area is top-notch. My tour guide, Chu, was wonderful (I?d recommend him to anyone visiting the area, email me for his personal email address). He grew up in the area and speaks most of the local dialects and English. He has loads of knowledge and I?d trust him taking me anywhere in the Sapa Valley mountains. At $4/day (although I think he should charge more) he?s well worth it!
Chu also arranged a homestay for me in one of the minority villages. Although I was disappointed by the ?tourist? feeling of the area (there were several other households in the village offering homestays) which detracted from the overall experience, I enjoyed communicating with the family.
After hiring Chu for 3 days strait and hiking over 35km I?m really glad I decided to stop in Sapa. If you are going to Sapa, hire Chu! (I promise you, I?m not getting a commission 
).