From the first time I traveled to Europe I have collected coasters. Coasters are a great souvenir, often they are free, they come in different shapes and sizes, they are easy to pack and when you get home they are a useful item.
Stock your bar with the coasters from your travels. When you are entertaining use them and they become conversation starters. People ask about your trip, what pub, restaurant or resort you were staying at. I like having souvenirs, and to me the best ones bring back memories and also serve a purpose.