We spent 4 nights at Spiros place. The appartement is perfectly located in the picturesque village of Kastro. You got everything you need within a distance of 100m, several restaurants and breakfast place, mini Market... The place is an authentic cycladies house where you are happy to find some shade and cool air after walking in the bright and hot sun. The apartment is very conveniently organised and furbished, and pleasantly decorated. Spiros is a great host, very responsive and helpfull. He booked for us a taxi to pick us up from the harbour witch is 15mn drive from Kastro.
There are several beautiful hikes starting from Kastro... if you don't mind climbing lots and lots of steps ;-).
Renting a car or a scooter might be useful to discover all the beauties of Sifnos.