There are hundreds of songs and poems written about Hanoi in autumn, which talk about the beauty of Hanoi and I agree with these poets. Between September and November is the best time to discover Vietnam, especially Hanoi.
Walking along the streets and lakes and you can enjoy breathing in the beautiful sweet flavor of Hoa Sua flower(Hoa Sua means Milk Flower in English) and the willow trees hanging low. Autumn turns Hanoi into a really romantic place. It affects the people, too. Lots of young couples walk together or sit down alongside the lakes to exchange their kisses.
The weather in Hanoi during autumn is cool, a little bit sunny with a nice breeze that makes everyone much more active after the long hot summer. I love hanging around Hoan Kiem Lake and Truc Bach Lake on these days, looking at people and taking some photos or sitting down with a beer waiting for the sunset.
A warning - don't breath the milk flowers in too deeply as this might give you a headache.