Not sure if this is applicable since it's technically about places out of Ningbo, but it might be considered relevant.  My personal favorite place is Wulongtan, followed by Xikou probably. I guess i got a thing for waterfalls.  They're more expensive and longer drives than a trip out to say, Dongqiang Lake, but are also more removed from the city and cleaner.  When all I see is the murky downtown rivers every day, it's a relief to meet blue pools and clear streams.