Taxi from Noi Bai International Airport to Hanoi city center (Hoan Kiem lake) the fare would be around VND 350,000-400,000 or USD 15-20.

Bus from Noi Bai International Airport, visitors can take the following public buses to get to Hanoi's inner districts: 86 (to Hanoi Railway Station), 07 (Cau Giay district), 17 (Long Bien bridge), 90 (Kim Ma street)

TAXI in Hanoi City
Hanoi's taxis have meters, but not all drivers like to use them. Working meters cost about VND 10,000-15,000 for the first two km, then about VND 8,000 per succeeding kilometer.

GRAB BIKE AND GRAB TAXI
Grab is an app like Uber, but it offers more options, such as Grab motorbike rides. On just about every street you can spot Grab motorbike drivers in their notorious green jackets and helmets.
Grab is the best option to choose to avoid scam in Hanoi.

ELECTRIC BUS
Electric Bus tour is actually a pretty good way to get your bearings in the city.
Its network contains of 14 stops in the Old Quarter and around Hoan Kiem Lake.
Nothing really beats haphazardly discovering the nooks and crannies of the Old Quarter by foot, but if you’re feeling a tad lazy, the hop-on hop-off bus is worth considering. The main;departure point;is the northern end of Hoan Kiem Lake, and there's another departure point at the Opera House. A full journey around the Old Quarter takes around an hour, with a recorded English-language commentary inclusive.

CYCLO
Cyclo are Hanoi's bicycle rickshaws. Passengers ride in the front cab, while the driver sits behind the passenger. Cyclo cabs are made for two passengers and are ideal for exploring short distances within Hanoi's city center. Ride them only if you're in no hurry, and if you don't mind the fright of seeing Hanoi's traffic whiz right in front of you.
A ride in a cyclo should cost you about VND 100,000 (USD 5) for an hour's ride.

MOTORCYCLE TAXIS
Travelling via motorcycle taxis is a quick and inexpensive way of getting around in Hanoi as you can find one in most parts of the city, particularly nearby hotels, shopping streets, and tourist attractions.
Rides within 2 km should be about VND 20,000-30,000.

RENTING A SCOOTER
Renting a scootermay be an option if you want a little more flexibility to your travel around Hanoi. Many guesthouses or hotels can get their guests a motorbike to rent for about USD 8-10 per day.