In Mykonos, Princess had a shuttle at $7 per person return to town that took all of five minutes. Or other people just walked but there is no sidewalk really. Just a slip really beside the road. So beware. Nice picturesque, great photos of windmills and rocks, and houses hanging on edge of water. Very much like Venice looking back at my pictures.
Santorini, we took te funicular up the mountain. Get off ship early my advice as soon as it docks and then you can walk along the ledge and grab yourself the most wonderful view of blue water as you make your way past all the little houses perched on the cliff on your walk to Fira. Then stop for a little breakfast (pricy!!! My son's Diet Coke cost us $4Euros)Anyhow, it was an experience he remembers. Then we grabbed the bus to Oia as it passed us by and we took a cab back. Oia is wonderful once again. Would have loved to watch sunset from that place but of course our ship awaited. Best to do your walk first thing in the morning before the cliff gets crowded.