Lollapalooza is a great festival site that already uses flat design. Not only do they use flat colours but they seem top match what google use in their material design.
The home screen again has announcements about the festival;
The tickets page gives the uses colour to make the different tickets stand out the best;
The lineup page is again, just like most of the others, just a list of names, lacking any interaction at all;
There is a page about the festival and it’s history, it is basically a blog with articles on it;