For the ones really fit, I recommend the three day trekking to the Fan Si Pan mountain.
Sapa Trekking in the Mountains
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.
Sapa Silver Falls or Thac Bac
Eight miles (13km) from Sapa is the Thac Bac Water fall (the one in the picture). You can get here easily on a motorbike.
This is one of the region's major attractions. It is possible to climb to the top of the waterfalls. From there, there is an excellent view of the waterfalls.
If you come here, follow the road and you will get to the Tram Ton pass.