I'm starting to entertain the idea of afterschooling maybe once/week...but I have mixed feelings about it. I'm not really seeing any "red flags" that he's bored out of his mind and he does tend to challenge himself in his free time.
Here is what our afterschooling involves for us: it very low pressure, generally dinner-time discussion of something that DS5.5 or 2.5 finds interesting. That could be talking about cell division (nice way to get back into exponents), it could be talking politics (DS2.5 lights up anytime he hears Obama or McCain mentioned on NPR), doing an experiment to see how long ice takes to melt, or whatever. You get the idea. During the school week, this probably only happens half the time because DS5.5 is pretty exhausted.
The other thing that we do from time to time is bath crayons. I know that this could come out sounding like hot housing, but really when we do math or biology in the bathtub, it's usually cited as DS's favorite part of the day (part of our night time ritual is to name the favorite and sad parts of the day). Using bath crayons in the tub after getting washed up lets us do all kinds of crazy things... borrowing, carry over, diagrams of the kidneys,

During the weekends we do try to do something educational. We're lucky that we're in an area with a bit of history and near NYC and Baltimore. Since that does take time time, money, and planning, there are plenty of weekends where we just chill. We make available workbooks for 2nd and 3rd grade that DS just likes to do on his own from time to time. And we DVR educational TV like CyberChase and Between the Lions.
That's most of the extra-circuluar activities we do... (except for some other things in biology which I consider a special project of DS's.)
Jool, what are the mixed feelings that you are having?
JB