Wedding guests write notes and slip them in whatever year bottle they want. Open it on your anniversary that year.--this would be especially special for the later years where maybe some of the people who put something in those bottles passed away.
NUMBER your rsvp cards because some people write a no where they should put their name and you have no idea who it is! Once you get them back in the mail, go into your excel spreadsheet and mark them coming or not coming. Then you also dont have to worry about losing the cards and you can easily tell who still owes you one