My wife and I had a Winter wedding--not an "It's a Wonderful Life" theme, but it was very vintage style.
A neat idea that we had that might work for you was, instead of having a guestbook, we reproduced old 1930's Wintery postacards and placed them by an old mailbox.Guests filled them out and dropped them in the box. Everyone loved it. We mocked up the postcards with our names and wedding date on the front. Perhaps you can do the same, with a "Greetings From Bedford Falls" type of theme.