Vasiliki was very helpful in providing a wealth of information for me to explore Santorini.
If I had more time instead of just one day I would have stayed longer.
I definitely recommend renting an ATV from Moto Explorer Rental (tel. 22860 32697) that is a couple businesses down, from the corner bus stop, and ATM machine.
I was able to rent an ATV for $15 Euros, possibly due to the low season, and owner is very honest, he said I do not need extra insurance, and does not believe in up-selling more for unnecessary insurance. Going around Santorini to Fira is pretty safe as long as you stay within the speed limit.