My friend Heather told me about this idea, and it's so simple, yet so sweet. I began my own collection last year. I love fresh trees, and I purchase one each year. I asked the guys bagging up my tree this past year to give me the cut from the bottom they always saw off to have a 'fresh' cut for when I take it home drink water. I wrote the year, who was there, and where we were (there's a bit of a 'code' for the locations, but I'll know what they mean).
I also used some mod podge, and I put a picture from Christmas on the reverse side.
I sprayed it down with some waterproof sealant (that I use to make coasters).
I may eventually drill a hole in the top and turn these into yearly ornaments, but for now, I'm going to place them in a bowl, like Heather does. I loved exploring her collections of various sized trees, and the memories she inscribed on the wood using a permanent marker.