I love this idea. An envelope guest book. You just take a store-bought scrapbook/album and glue envelopes into it. Lay out colored pieces of paper for guests to jot notes down on, and then they can place them in the envelopes.
Instead of picking just ONE Harry Potter book to represent your love of the series, make a complete seven-book set of adorable Potter-inspired DIY cover ornaments that will make your tree look magical.
Love books by color and using law textbooks to anchor in our house. Also, we are totally that couple that would create bookshelves in some nook of our house like this. Great idea for making the most of your space, over the door shelves.