I really love playing with APIs and the data you can tap in to is often very impressive.
The key data to draw from 4sq is the user check-ins to see where a person has been. From this you can then build upon the data with various visulisations or validations, such as:
The most important part is establishing a method to perform the API calls and properly handle the data returned.
The official Foursquare developer portal has a lot of good information to assist with this, but below is an open source app written by myself.









