i've been living in Hanoi for about a year now, and I'd definitely recommend buying or renting your own bike to do your trip. It's easy to get a bike in Hanoi, and you can ride from there to places like Ninh Binh, Cuc Phuong, Tam Dao etc. You could also put your bike on the train and take it up to Sapa, or ride it there! The best place i know to rent/buy bikes in Hanoi is Phung Motorbike, which is at no. 13 Ngo Huyen street (near the cathedral).