Besides Sapa (which is a very nice place and highly recommendable itself), there are other nice trekking routes in Northern Vietnam as well. I recommend you go to Pu Luong, Mai Chau, Cat Ba (which has Lan Ha Bay, kinda similar to Ha Long Bay, but much less crowded!), Cuc Phuong National Park or Ba Be National Park.
Navigating around these areas can be quite hard if you don’t have a local tour guide. Last time I went with the guide from Hanoi Voyages. It was a lovely journey, the guide was super funny and the experience was beyond words. Many other travel agencies offer trekking tours as well, so I suggest you can have a look for yourself and decide!