spray paint measuring tape and keep track of kids heights at different ages. Takes up no space and can move with you. I would paint the back side to mark so you can still see their height without having to remeasure your measuring tape.
Scripture Calendar - Love this idea! If anything you'll memorize 12 verses each year. Could add a numbered blank list on each page to write things you were thankful for that day (fill up the whole month)
What a neat tradition for every year? A Day In My Life: Birthday Interview. I like that its simple questions, like the handprint too. And this is simple.do one each year on their Birthday and put in a book!