The most popular transportation in Vietnam is motorbike. Not surprisingly there are lots of "Xe Ôm" which means motorbike taxi on the street. Everyone can afford to travel this way, reasonable price and they are present everywhere.
For 10,000Vnd that you can go for few kms which would costs you about 40,000Vnd in a taxi. The good thing about this service is you can see scenery but be careful which driver you choose. Lots of drivers in Hanoi drink rice wine during the day and they might give you a shock with the way they drive. Try to see how good you are in bargaining.