I spent 5 weeks travelling from Hanoi down south to Ho Chi Minh, then up the river into Cambodia. That was in Jan 2005.
One of the best and most memorable trips of my life. Hanoi is a much better city than HCMC – more interesting and less damaged by globalization. From Hanoi def get a trip out East to Ha Long Bay and take a boat trip around the Islands – very touristy but a great sight – I took an overnight boat which meant we got to experience the phosphorescent seas.
In Hue, get to the 'Cafe on Thu Wheels' and take a moped trip around the ancient temples of the area.
Hoi An is very pretty place (but touristy) and a good place to get custom made clothes.
Nha Trang is pretty naff – it was turning into Vietnam's Benidorm when I visited.
Cu Chi Tunnels outside HCMC are worth a visit to see how resourceful the Vietcong were.
Take time to explore the Mekong Delta south of HCMC as it is such a contrast to the mountainous north.
Tip: when getting a ride anywhere (on the back of a moped or taxi) – agree a price first. Cyclos (pedal rickshaws) can be a real rip-off.
Enjoy it – its ace.