Visitor Counter
This project is to develop a visitor counter for the Physics Museum.
Regarding the visitor counter: Other people have generally used either a photo sensor or PIR motion sensor. They both have some limitations, but might be good enough for this purpose. Many PIR motion sensors have a field of view that is too wide. One person got around that by attaching a tube to the end. Photo sensor/IR sensor would also work and it might be possible to make it bi-directional by using two sensors (one at the beginning of the entryway, and the other slightly further away. If you want a simple count though, you could have a single photo sensor and divide by two to just get the number of visitors. With both of these solutions, you'll have trouble getting accurate results when a number of people enter at the same time.
These are two very doable and cost effective solutions that other people have done: http://www.dbpharrison.com/general/pircount/
http://www.instructables.com/id/Room-Visitor-Counter-with-Relay-Control/?ALLSTEPS
Both of these would be done using an Arduino and supplies that are readily available on Sparkfun and at the Physics Garage. To add to their projects, it would be a good idea to use a WiFi shield with the Arduino. You could then have the data collected post to a database. From there you could post that data to a website/local server and generate stats about the number of visitors and when museum use is the most active.