In any interactive installation environment there are plenty of way to get user input / data that will drive your visuals and give the audience the feeling of connected-ness with your installation. One of the more interesting ways that I have done recently is by tracking people and their movement around the space.
This concept is not new by any means and has actually taken many different forms over the years (Blob tracking, Motion Capture, Blacktrax). However these systems can be either WAY too expensive, a pain to set up, or not able to give you any reliable data.
OpenPTrack is open source person tracking software which works on a distributed network of depth cameras. The system constructs a streaming point cloud image and then separates the forms of people to then track around in the space. Then, like most tracking systems, it spits the data out onto the network to allow other programs to use it. Go check out their website to learn more.
I worked with them to develop two TouchDesigner components for taking in the streaming data. You can find them at my Github repo. There is one TOX file which streams in data and parses it with Python. The other is a CPlusPlus Chop DLL which works much faster than the TOX when tracking a lot of people (20+).