For the ones really fit, I recommend the three day trekking to the Fan Si Pan mountain.
Mount Phan Xi Pang, as locals call it, is a 3500 yard high peak (3143 m). During the trek you will have the chance to see how locals live. And even sleep in a local home. You will need special permits to do a home stay. For this reason I recommend reserving one of the trekking tours in Hanoi or Sapa.
The best season to do this trekking is in summer. In Winter conditions may be too tough.
I did not do this trekking, so I cannot give you my personal impressions. All I can say is that if you like the picture on this page, you will love the Farsipan trek.