If you like hiking you MUST visit Sapa near the Chinese border.
Truly, truly stunning.
Book a trip from Hanoi - the one we took comprised travel there and back pn an overnight sleeper train, hotel for one night and a night staying with a hill tribe family, all food included for the princely sum of about £50 each!
Halong Bay is a must - again, book from Hanoi and stay overnight on a traditional junk.
Hoi An, a world heritage sight is a beatiful little town. Save some luggage space to get some clothes tailored - we got 4 pairs of trousers for about £18!
Hanoi and HCMC (Saigon) are ok but all S E Asian cities are much of a muchness. I'd advise to just use these as departure points for the more interesting parts of the country.
If you need any more specific info feel free to ask.