"Stones and Rocks DIY Wedding guest book". Guests write messages to bride and groom on rocks with Sharpie instead of signing traditional guest book. This would be cool to put in giant glass vases on either side of the fireplace once the wedding was over.
Assign each table a different anniversary year (i. table 1 would be anniversary year etc.) and let the guests at that table write notes to be opened on that particular anniversary year (a unique alternative to a wedding guest book).