I've seen the matching shirts thing before - it looks really cute.
My favourite reveal (long after I'd got my big sis, and my little sis) was a candlelit circle. Each little sis would have received her last present of big/little week, and inside the card was half of a poem about sisterhood or friendship. She would close her eyes and be led into the centre of the circle, then her big would come stand back to back. The little sister would read the first half, then the big sister would read the last half, then they'd turn around and there'd be much hugging and celebrating.
I thought it was super cute.