What a great idea! It gives guests a cool souvenir from their time and makes them an advertiser/brand ambassador for your property. I personally love collecting stickers or magnets or coffee cups from the places I visit, and it's even nicer if I don't have to buy it 🙂
One thing we do that has seemed to resonate with guests is seasonal/holiday decorations. They appreciate the coziness and festive feeling it brings to the space, and we have had guests return several years in a row (plus write nice reviews and make recommendations) as a result.
Another thing we sometimes do is to leave a gift basket created by a local small business owner and filled with local items. Guests feel valued plus have a talking point and perhaps goodies to share with friends and family. It also serves to introduce them to the local area and encourages local artisans/vendors/business owners to think of our property when making recommendations.