My laptop gets used by many people for many things, so it's handy to know what people are using it for. For example, it was recently used by a work experience student in our office who decided to search for porn on the internet -- a serious problem seeing as it is my laptop, and my job on the line. Everyone probably knows that one can use the Recent Items feature in the Apple menu to see what's been used lately on the machine.
What people often don't remember is that there is also a Recent Items menu in most programs. If you're in the habit of letting others use your machine, when you get it back, check the Recent Items menu in the Apple menu. If it has been cleared, it pays to open Preview and QuickTime and check their Recent Items list for any non-desired activity.
This is not foolproof, of course, as there is a Clear Menu option for most Apple-produced products. Nevertheless, although probably not the most effective way of monitoring other persons' activity on your computer, for simplicity and ease, this should be your first stop.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006083119483020