Hanoi can be a charming city and has a totally different feel to Saigon. It's very likely you'll have to spend at least some time there, as its the main arrival/departure hub for Halong Bay, Sapa, and Ninh Binh.
People seem to have a slightly harder edge to them here compared to the rest of Vietnam - you'll notice it if you've spent time in Saigon or the provinces first - but they're friendly and outgoing; if you sit around in cafes or bars here, you'll be surprised at how many people approach you for a chat.
The city these days is quite sprawling, but its heart is the old quarter which is easily covered on foot - narrow, chaotic streets, plenty of places to eat on the street and great fun to wander around.
What to see, what to do?
Take a walk around Hoan Kiem Lake, or Lenin park, and watch the Hanoians exercising and relaxing.
Get your museum fix: we really like the Women's Museum - you can really appreciate the active role women played in the American war after a visit here. If you still want more, there is always the Ho Chi Minh Museum, or the Museum of the Vietnamese Revolution, and more...
Check out a performance of the famous Hoi An water puppets for some traditional Vietnamese music, then get lost wandering the old quarter and experience some of the great street food on offer.