Fa Men Si, Xi'an
A big temple that you need at least 2 hours to wander around. Note there are 2 important sites that you have to know, one is the pagoda that is the where the Holy Relics were discovered, the other one is a big building for the Holy Relics, however, the Holy Relics can be seen only 1st, 15th of each month for Chinese lunar calendar, and Saturday & Sunday.
The adjacent new site is too modern for my liking. We were unable to enter the pagoda as it was closed for renovation. Since it is so far from Xian you should come only if you really have the time