Sapa is located at an altitude of ~1500 meters. There is nothing to do in town per se. The lake around which the own has developed was not too clean so I stayed away.
Apart from trekking spots nearby, you can visit local villages(Cat Cat village is well publicized but your guide will know other villages to take you to), view the rice terraces, walk through the meadows for over a kilometer to Love waterfall and enjoy the mountain scenery. The one other place to visit is Fansipian peak via a cable car.
The locals, especially the boys, seem very comfortable handling snakes, which abound in large numbers. The one state in India I have seen something similar is Assam.
Fansipian peak
The Fansipian cable car station is located ~ 3 kms away from Sapa town. You can walk, take a car or the train. The Fansipian peak is 3143 m above sea level and is the tallest peak in the region. For the more adventurous, there are trekking routes all the way to Fansipian peak.
The cable car travels a distance of 1.4 kms and takes about 15 minutes. You get great views of the mountains as well as rice terraces. The cable car does not go all the way to the Fansipian peak but stops short by 600 m. A Funicular takes you to a higher level after which you need to climb few steps to reach the peak.
The view from the peak is good but remember, the mountains here are not really high. But very good forest cover and hence looks delightfully green.
The food court at the cable car station has limited vegetarian options. Better to carry packed snacks if you are prone to munching every hour. Overall, 3-4 hours is adequate.
Trek to Love Waterfall, Spa, Vietnam 
View from Fansipian Peak, Spa, Vietnam 
Rice Terrace, Sapa, Vietnam
