Depends on the sickness.
If I'm too sick to go to school, I usually treat the day like a day off....except no friends, lots of liquids, (sprite, ginger ale, gatorade, soup) and even more lazing around than usual......games, movies, cpu, games, more games.....I barely ever take naps.
If I have a flu/headcold type thing (you know- where your constantly cold, achy, weak, maybe even nauseous) I'll take a lot of hot showers/baths throughout the day. It's amazing how much better they can make you feel, even if just for an hour or so.
If I'm puking constantly I'll go the entire day without eating a thing and always make sure I'm near a toilet or have a pail with me.