This is something creative and simple, and a keepsake you will cherish forever!
Simply purchase some sort of scrapbook and fill the pages with envelopes. (what ever color theme you desire)
Guests can leave words of advice, and sign their names on the envelopes. You can also fill the pages (rotate every other page with envelopes) with photos of the bride and groom, some of your favorite quotes, memorabilia from dates (movie tickets, etc.).
This is an idea that can be completely personalized, and made just for you!
It is also something that the bride and groom can look through year after year!
(Another fun idea is to ask the guests to leave notes in the envelopes answering where they think you will be in 5, 10, 20, 30, etc. years.
Having different pages be different timelines)