I really enjoyed my first visit to Hanoi in January. I recommend visiting it; of course, am not sure how many hours you'd have flying in and out from BKK in one day. The center of the city is around the opera, adjacent to the Hanoi Hilton Hotel. On just a day trip, you probably don't have time to see the water puppets - a great shame, because it's definitely a unique Vietnamese art form. There's a great cultural museum on the outskirts of Hanoi; I really enjoyed learning about the many cultural groups of Vietnam.