As a couple, give Donkey Ride (Fira Steps) a miss.
It's a piece of old Santorini, but animal-welfare groups have documented overloading, heat exposure, and injury, and the route is unpleasant for the rider too (steep, slippery, and smelly). The honest verdict: skip it. Walk the steps or take the cable car — the 'experience' isn't worth the ethical cost.