I don't have any of these stamps issued by Ireland's An Post, but I think they are really well done. They celebrate the traditional five senses. Each one can be scanned with a smart phone to reveal more information, and each one has an extra feature connecting it to the sense it represents.
Taste - the gum tastes like strawberry - maybe all gummed stamps should have a pleasant taste!
Touch - this stamp incorporates thermochromic ink which changes color when touched.
Sight - the stamp is transparent, allowing it to take on the color of the underlying envelope.
Hearing - using a technique I've never heard of, thermography, when the stamp is rubbed it will emit a sound.
Smell - the final stamp has the fragrance of mint.