To count as a day trip you must be able to get there and back in one day.
My favorite wilderness day trip is to Nandu River in Haikou on Hainan Island, south China. It is just on the edge of the city, but once you descend the riverbank it is surprisingly peaceful. You probably will not see anyone, except for the odd fisherman.
You can see photos on my travel blog: http://www.haikouhostel.com/hainan-travel-blog/131-kayaking-in-haikous-nandu-river