It might not be possible to have too much fun, but it's definitely possible to have to much Phun. Phun is a "2D physics sandbox". What that means is that it allows you to draw shapes (circles, squares, or arbitrary freehand shapes) and then lets you bang them together using simulated physics laws. Check out the video at the link above for a demonstration.
Phun is exactly the kind of computer "game" that my brain adores. I used to play hours and hours of The Incredible Machine, which is a very similar concept. For me, at least, Phun is seriously addictive. It's like crack for Newtonian physics lovers. The windows version has been out for a while, but they just released the Mac version about week ago, and I've been like a kitten in a tub full of catnip ever since.
Oh, and it also lets you play with particles that mimic liquid water. Here are two of the more interesting creations I've made. The first video had to be sped up a lot because my computer couldn't render all those water particles very fast. For the second video, I added some sound effects. It'd be cool if Phun did sounds by itself.
And, of course, there are thousands more videos like this on you know what site.