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/
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.