Good call on using foursquare to do the work. I'm not sure if you can use this source to include the best times of the year also, but this would be ideal. When people ask themselves (or others) "when should I visit?" they are often referring to "what time of the year should I visit?"

For a 24 hour job, I'm wowed. I can see a lot of expansion potential, notably in adding location (by country, state, city) and categories (i.e. breaking it down by museums, theaters, parks, etc.).

Consider making the graphs into a widget for local tourist, hotel, and city pages.