Anywhere around the West Lake is going to be beautiful at night, but one particularily popular spot is xihu tiandi. That spot has lots of dining options and cafes like Costa and Starbucks as well as Haagen Dasz. These venues are nestled in between bamboo and connected via cute little stone paths. Just a stone's throw away is the glorious West Lake, and just outside the bambooed area with restaurants, you'll find an open space with a fountain and lots of people hanging out. I really liked this spot when I visited Hangzhou.

Any of their shopping malls. China always has interesting places to go no matter where your at. The lakes at night once lit look pretty nice just to go take a walk. With all the neat buildings they have I would say just about anywhere. But if it were me probably the shopping areas cause it's always busy and something is always going on.