For your health:
Do not eat somethings from the street food vendors those cut fruits and some cakes that those don't have anything to cover. Street food in a small food shops are OK. If you are worry about MSG you can ask for no MSG ( không mì chính)
By bus: BUS EXPRESS 86. From the airport .Station T1 for domestic . Station T2 for international. The bus comes every ~20-30min. The price is 30,000vnd for one ticket.
From the airport to the Old Quarter: For 4-seat car is about 250,000vnd and for 7-seat car is about 350,000vnd.
From the Old Quarter to Noi Bai airport: For 4 seat car is about 200,000vnd and for 7-seat car is about 250,000vnd
How to travel politely in Vietnam: If someone tries to sell something to you just walk away don't need to say anything, that seems impolite but it's not but if someone asks you to talk you can talk if you do not interest in you can say no, thank you. ( không, cám ơn). When you are walking on the street someone want to fix and clean your shoes do not do that, they will charge you a fortune, if you do, bargain.
If you buy something you should need to know how to bargain. Just let you know that in Hanoi has a lot of shoes and clothes are very cheap but those are fake but sometimes fake products do maybe good. The original products are cheaper if you buy outside of Vietnam. I just don't want you to be disappointed.