Make sure that if you find a particular project interesting you write down whose it is so that you can talk to them, or you will forget. Meeting with professors is the most important event. This is where you get to find out if a professor is someone you would actually be interested in working with. This will go smoother if you already have some idea of what their research is. The advisor relationship is the most important one that you will form in graduate school, and you should think about it very carefully.
Demos are probably the second least important of the events.
You will get to see the cool stuff that is going on in the department, however, you can probably get a better idea by talking to the professors or grad students. While it can be fun, I would only go to the demos if you do not have anything else to do. Talking with grad students is very important for several reasons. The graduate students will most likely tell you what is actually going on in the department. They can probably answer most of your questions or point you to the proper person to ask.