Once Agatha was standing I wanted to start marking her height but I couldn't quite bring myself to mark up the walls nor did I really love any of the growth charts I've seen for sale. The solution: a wood board and a sharpie. An aged board would have been way better but I didn't have one of those handy.
Since I had lots of room, I included boxes to mark other interesting milestones and firsts for her. The type of things that I might forget if I didn't write down and she might find interesting: countries + cities visited, halloween costumes, first words, schools, books read, etc. I tried not to make them too baby centric so the board stays relevant for more than just the first few years.
I added a coat of Danish oil thinking it would darken up the wood but I don't love how it highlighted the grain. I probably should have just left it bare or stained it prior to writing on it but live and learn.