If you homeschool... and you have to count educational days... don't think because someone's sick you can't count it. You can...
... read a great story aloud. It doesn't have to be something new - an old favorite may be easier for sleepy listeners.
... watch some tv or a dvd. It can be something "officially" educational, say, an episode (or a few) of Planet Earth. It can be marginally educational: Little House on the Prairie or an American Girl flick. It can be a classic: Mary Poppins or Chitty Chitty Bang Bang or Heidi. (Obviously my ideas are mostly for younger kids.) Do you think all the moves kids watch in school are truly educational?
... draw - maybe make Treasure Island maps - while listening to music.
... play some card games, improve those strategy, math, and social skills. (Some people have to be trained out of being a poor loser.)
... look up historical events of the day and discuss some of them while sipping hot tea and lounging on blankets on the floor. Here's one source; there are many others.
... play computer games or browse interesting sites, like National Geographic videos.
... do a little skills review with workbooks, such as the Spectrum Test Prep series. (Of course there are others.)
... write stories on the computer or in a notebook.
... make paper airplanes or paper dolls; work on a model kit or do some sewing.
... do some more reading; remember that audio books count too!
You might find the kids learn more on a sick day than on a "regular" day.
How do you manage sick days?