We put together a folder that contained a dozen or two pictures. We figured that they were small and if they didn't want to flip through them all, then didn't have to. Generally these were pictures we took at the time because we that was DS was doing was 'interesting' but we were pretty clueless on the fact that he was out there. So we had pictures from 3 years of age to 7 (the age DS was when we applied). By age 5 we suspected he was at least HG so we knew to take a picture every now and then. Otherwise no one would believe us :-)

I think we also include a few worksheets, but not too, too many. And I had already made a couple of home movies to share with my folks so I put them on a thumb drive and sent them in.

It was probably overkill, but that's what we did. I think anything that shows your DC working 'several grade levels above' a typical child would be fine. But take that with a grain of salt.

Good luck!