How to easily kill hours of your life

Allright, everybody was talking about this Minecraft, so obviously I had to check it out. It appears as if it is in it's alpha stage and you have to pay to play the actual alpha, but there seems to be a pre-alpha version you can try and mess around with. While doing that my clock just skipped three hours ahead, even though I was supposed to do something completely different.
There are no objectives or goals in minecraft yet, but the developers promise for this to change soon.
I do believe, however that this is not necessary. You run around in a randomly generated 3d world and can create and destroy the blocks that this world is made up from. Very easy concept. Apparently in the alpha version you can also build railroad tracks and stuff that actually works. Sweet.
So yeah, all I did the last three hours was run around and build stuff. Check this out:

First, I built a little temple

Then this menacing tower...

And then added an underground library.