Oops! Missed giving a warning on this one. Already half-over, but there is another day. The Hakka Festival at the Hakka Culture Park (formally the Transportation Museum for those who had kids around the same time I did).
Chinese timetable is here.
Official English website here.
I haven't been here for many years, so can't really add anymore insider information.
You can park at the water park carpark.
Address : No.2, Section 3, Tingzhou Road, Zhongzheng District, Taipei