As jaklumen has already mentioned, since my daughter started school, I have been providing her with an activity a day in one of five subjects which are, in alphabetical order, Art, Math, Reading, Science, and Writing.  I just thought, since my husband brought this up, I thought you all might like to know a little about what I'm doing with these.

Purpose:  Encourage my daughter's imagination.
Typical Challenge:  I suggest a general topic and ask her to draw a picture to match it.  Sometimes, I will provide her with a new medium to do her art in.
Future Considerations:  Sidewalk Chalk.  A picture with her favorite animal.

Purpose: Kick start my daughter's math skills.  Get her interested in math early on.
Typical Challenge:  Before this year, my daughter has only counted, so I bought her a workbook with some simple first grade math problems in it.  I always help her when she has a Math Challenge.  However, if she brings home Math homework from school, I will usually put the workbook aside and make her homework the challenge.
Future Considerations:  Counting different colored marbles.

Purpose:  Basic practice at first, then, later, to increase her reading level.
Typical Challenge: At the moment, most of my daughter's reading challenges involve reading to her baby brother.  It's sort of a "killing two birds with one stone" thing.  My son gets read to, my daughter reads.  Everyone's happy.
Future Considerations: Some easy chapter books and other first readers with fewer pictures.

Purpose: To rekindle my daughter's sense of wonder and boost her sense of observation.
Typical Challenge:  With this subject, I typically leave out a number of kitchen supplies and instructions how to use them for an experiment.  There are more than enough kitchen science experiments that can be done.  At the moment, we're working with just the basics.  This subject, in particular, seems to be sparking my daughter's interest.  She tends to look forward most to challenges involving science.
Future Considerations:  Picking an ice cube up with string, making a rocket using a soda bottle and some baking soda, making rock candy or salt crystals.

Purpose:  As with the Reading Challenge, give my daughter an opportunity to practice and possibly improve her writing skill.
Typical Challenge:  My mom loves to get letters from her.  So, just lately, most of her writing challenges have involved writing to her grandmother.
Future Considerations: Write a sentence or two into her journal that tells how the week was.