Before I moved to Vietnam I took a 5 day motorbike tour with Explore Indochina. It was awesome. Five days in the Northeast region of North Vietnam. Breathtaking scenery and remote locales. You can check out my pics here.
I was reminded of the trip by this video from Explore Indochina, below. While I didn't traverse raging rivers, I did have plenty of adventure. I want to do another tour, this time in the Northwest of Vietnam, including Sapa. Explore Indochina is a reputable outfit and I had an excellent experience.