The highlight of my time so far in Southeast Asia was the Easy Rider journey I did from Dalat to Hoi An.
I had the crazy idea to buy a motorbike in Saigon and ride it all the way to Hanoi, so the price of the Easy Rider program was a bit steep.
It's usually $75-85 per day, although I got it for $50 since I had my own bike (my stepdad would be so proud of my bargaining skills!)
My guide was amazing - Mr Binh was full of knowledge about the Vietnamese war, local customs in the minority villages we visited, and Vietnamese culture in general.
I cannot recommend this experience enough - the places you see, the local people that you meet (and can actually talk to, thanks to Mr Binh), and the things you learn...absolutely fantastic!
I think we saw about 2 other westerners each day, typically also on bikes heading the opposite direction.
If you're looking for the real Vietnam experience off the beaten track, don't think twice - just do it!