An alternate title for this project might be "How to spy on naive users while enjoying a latte". ;-)
Access points in public places are usually unsecured. Using Wireshark or another program you can capture all the packets. You can then sort the packets by user (using the IP address), then further sort the packets by protocol (say HTTP for web sites), and then by the web site. From this you can form a profile of each user by the primary task they are doing (say Facebook) and the secondary tasks (other web sites, Skype, etc.). You can then form a profile of the users in the coffee shop which you could use for various purposes, such as amusing yourself, advertising, stealing passwords and identities, locating dissidents and arresting them, etc.
An interesting thing to do might be to classify the HTTP and HTTPS traffic to see how many people are concerned enough about their information to use secure connections. Conducting a scan in various public places might yield some interesting insights. For example, are people near the local police station more or less concerned about security than students at university? While HTTPS traffic is encrypted, the IP addresses are not, so you can still form a profile of what people are doing in general, while not being able to see the content of their messages.