We loved this house in Cala Serena. It is set in about 50m from an impressive cliff-edged coastline. (This is something to be aware of if you have children. However, I'm not trying to put anyone off with this; even with young children, the house is far enough away from the cliffs - unless, that is, you have a seriously adventurous toddler!)
The house is 5-10 minutes walk from Cala Serena, which is a nice little beach, that does not seem to get overcrowded.
The house is small but cosy. Its adobe décor is lovely, and the house has a sort of hollowed-out-cave feel to it (I mean that in a good way).
The house is walking distance to the tourist buzz of Cala d'Or, but far enough from it to not feel part of it (which is a positive point, in my book).
Cala Mitjana and Sa Nau are a bit farther away, but we preferred beaches farther afield. Driving (a fair bit) south-west to Santanyi, you find wonderful beaches such as Calas Santanyi, Llombards and S'Almonia; a similar drive north-east takes you to Cala Varques. (These last two require a bit of a walk.)
Air conditioning in the house makes it easy to sleep at night.
The house has a good little kitchen (but be prepared if you want to grill or cook anything - the oven/grill seems to be very low-powered).
All in all, great for anyone looking for a bit of peace and quiet in a great area.