I bought the WDW Passporter for my last trip, and it's acted as a wonderful scrap book for pictures, tickets, and memories in general. So, when I miss it, I go back to that.
Also, when I was in 4th grade my father and I started the tradition of writing a Disney Diary. 7 years ago I saw it in staples and it had disney characters all over it! So we keep a log of how many times we've been on ToT, the funny stuff that happens, who we bring, the people we meet, the restaurants we eat at, all the awesome cast members, and everything. It's turned out to be a nice book i can look back and watch myself grow up in disney! I highly recommend it. It's inspired so many new traditions too! Like i read it and say, "hey that was fun, let's do it again." And the next thing I know, we've got yet another thing we have to do every visit. It's a perfect momento--cheap too!!