What Can You Do to Improve Your English in China?

If you haven't planned anything in advance, I'd say your best bet probably would be to just go to a park and hang out and look like a foreigner. You most likely will have people coming up and wanting to practice their English with you. One is meetup.com, which is a site where you can find people with all kinds of different interests who have arranged meetings. Also couch surfing. A lot of people there don't necessarily want to host someone either, but they're more than happy to meet up for a coffee or chat.