You can send updated ACT/SAT scores -- those would be sent directly from the test organization.
You can send via snail mail additional accomplishments, such as being elected team captain, being elected to NHS, achieving a sports distinction (e.g., "All-District"), completing Eagle Scout, etc. The accomplishment should be somewhat significant -- not merely that you joined a club.
If it is a school-related activity, it must be certified by a school administrator. If it is a non-school activity, provide objective evidence to support it.
Seems my email and the above crossed. Good that we are (somewhat scarily, I must confess) on the EXACT same page.