I think we have our guest book alternative finally picked out!
I really loved the idea of the thumbprint tree, but I was worried about the logistics of it at the event.
I saw this idea on etsy and it's something so easy to DIY. It's a "Jenga" guest book where guests sign blocks. It's one of my son's favourite games and it's something we could actually enjoy for years to come. I found a company that makes the game without a logo (for cheap!) and I might paint them white just to tie in with everything else at the wedding.
I'm also going to place a little "instruction" card at the table saying:
"Marriage is a game of balance.
Sign a block and help us build our future."