Awhile back, my mother got Ben a calendar. A nice, big calendar with peel-off vinyl numbers so you can set it up for any month, and a slick surface that you can write on with dry-erase markers. Every evening we "do calendar" with Ben, meaning that we discuss his day and record some of the significant events. For example, you can see that he turned 18 months old on April 19th and had pizza on the 23rd. It's kind've like his journal, except that he's not actually allowed to use the markers, despite his oft-repeated requests. The plan is to take a picture of it every month before erasing it.
For those of you who want to do something similar, my mother got it at a teacher's supply store in Wisconsin. She's been retired for a couple of years now, but you know the saying -- you can take the teacher out of the classroom, but you can't get her out of the teacher's supply store.