I have posted my thoughts on not riding the donkeys elsewhere but while researching a cruise stop in Athens came across a great trip report by Hydrokitty
who reviews Santorini as follows:
Santorini: Ahhhhhh Santorini. What a beautiful place. Take the Thira (Fira) and Oia village tours. You'll drive the entire island, see spectacular views. spend an hour in Oia and the rest of the time (not enough) in Fira. Take the cable car down. Do not try to walk down. It's very winding, steep and is the same path the donkeys use so you have to watch your feet. Don't take the donkey because the donkey master will take you 1/2 to 3/4 down and then make you get off and walk the rest of the way because he'll have someone at the top waiting. If you refuse to get off or demand a refund he gets downright nasty and quite intimidating. Besides you get a magnificent view from the cable car.
Just thought I'd share it with you.