First you might want to check out math competitions.  Wikipedia has a list of many of these.

Or perhaps if you want to check some stuff out on your own here is a blog with a bunch of resources. ( be forewarned - not all of these are American, so you might encounter some, ... interesting English )

I recommend reading fun math and science books.  I'll try to grow this list but for starters these were a few favorites of mine from middle school:

Middle School Favorites

• Joy of Pi By David Blatner
• A History of Pi By Petr Beckmann
• Chaos: Making a New Science By James Gleick