Trekking in Muong Hoa valley (also known as Sapa valley) can be an amazing experience. An extended network of trails made by the hill tribes makes it possible to trek throughout the valley from Sapa town, to the top of Fansipan, through the jungle on the side of Hoang Lien Son mountain range, zigzag through rice paddy fields, cross rivers and finally end up in some far flung hill tribe village. Well, while this is possible, reality is that most travellers ending up doing a more touristy version, visiting villages along the valley road (like Lao Chai, Ta Van and Ban Ho) and missing out the best places the valley can offer. But this tourist-trail, which is easiest done with a local H'mong girl guide, doesn't necessarily has to be less amazing.
Exploring the corners of Sapa valley without going through a trekking company either on foot, mountain bike or motorbike is difficult, but not impossible. Some areas need permits, while other are totally off-limits - and the rules change like the wind. So seek information (from people who know and are willing to share) before venturing too far off the beaten track.