Just like other areas of Vietnam, motorcycle is the way to go. Everyone is willing to give a ride to the "rich Westerner," so you never have to wander far until you find a willing driver. It is cheap, usually negotiated down to $1 or $2 per hour. These guys also know where the big tourist sights are so they may take you places you didn't know existed or hadn't planned to visit.
I found it very convenient to get around Hanoi by motobike, but watch out for the terrible traffic! Accidents are very common and it is dangerous!
At almost every shop are 'motorbike parks' with guards who look for the bikes for some dong.
If the bike gets broken, don't worry: Vietnam is very service oriented. Everywhere along the roads you will find someone who offers you repair service. Also 'mobile filling stations' are easy to find.
Just the grade of service is sometimes debatable ;-)
Always lock your bike!
Motorcycle taxi is everywhere in Vietnam and is a good way to get aroud but please bargain with them and agree on a price before you start your journey.
wonderful, funny experience, on the motorbike of my interpreter Hien, into the Hanoi traffic, it's crazy...but so funny to be into the real life of Vietnamese people...