I am new here and am thrilled to see this thread! My ds8 is exceptionally strong on the math front and we, too, have been trying to figure out what to do about math once instruction at his school runs out. Fortunately, they are keeping him moving several grades above where he formally is in school (2nd grade). Funny--the principal said to us with worry: "Eventually we will run out of instruction for him." We were just happy that they can keep things going! We told her that we understand completely that we will need to take responsibility (or a large part of it, anyway) for his math education well before the end of middle school (which is when his current school ends). So anyway, it is great to see all of these options laid out!

A couple of very informal ways that we've been "enriching" his math education so far:

1. Discussions with my business savvy brother, who helped my son "make up" a company, figure out stock prices, rates of return, etc.

2. Discussions of probability as offshoots of card and dice-based games.

22B--FWIW from my very anecdotal sense of what's happening with college admissions (based mostly on talented kids of friends): It does seem as if the top-notch universities care a great deal about well roundedness and also are attuned to "padding" with extracurricular activities rather than pursuing extracurricular activities due to true interest. As others have said, your child could change a lot in the next 10 years! Don't know if this rings true to you, but our philosophy has been to keep our DS interested and learning (and interested in learning)--and we assume that things will work out. Maybe that is delusional...