Rather than explaining, you could try using hosts files to block what you don't want them to get to. I've tried Pollock's hosts file and it seems to do a decent job while not blocking useful pages. http://someonewhocares.org/hosts/

If it's going to be random, what's the fun of just day to day? It should change totally randomly and suddenly. Imagine watching your friends or significant others doing something as simple as pick up a beer bottle and they go from feeling like superman because the bottle is weightless to spilling it because of a sudden shift. Even better is the thought of the shift from "space gymnastics" to "WTF, I can't even stand up."