Count me in , but you should make something more interesting! not only dinner!

Bitcoin is anonymous. The merchant can see all the transaction information like your bitcoin address, TX Id that your bitcoins was sent from and other but they will never know what physical individual sent the money because they don't have any information about you, in case you need to provide shipping address if purchasing physical goods.

 If you want to prove that you sent the money adding a note to the transaction would be easiest solution probably.

This link should explain how activity works for you because It's newbie readme. You can get 14 activity per 2 week period maximum. https://bitcointalk.org/index.php?topic=177133.msg5106206#msg5106206