I agree this is a good "sampler", a 'snapshot' of the most-promoted Greek destinations. Don't expect much anything beyond fleeting impressions of the port area at most stops. Some tips:
OLYMPIA -- Site not close to port so u may have to do ship-sponsored excursion. WONDERFUL Museum & site.
SANTORINI -- skip excursion or not? Ship stops at Old Port. Excursion folks get first-dibs on launch; big delay!! for others. Only ways to clifftop are (1) a cable car with looong line, thus MORE delay; (2)ride on poor whipped donkey up 400 steps or (3) climb up steps. You're young; just do it! Go to Bus Area (map)
http://hotels.santorini.net/fileadmin/hotels_santorini/single_hotels/Aigialos/aigialos_map.jpg
Take bus to OIA http://www.santorini-web.com/map.pdf -- and whip out camera for famous Caldera-view pix! Bus back to Fira Town. Now 10:30 or 11. U have two choices about archeology wonder of Santo, Akrotiri (buried town), alas closed presently. (1)Arch Museum, by Cathedral - Great 3D exhibits, (2) Nokimos Conf. Center http://www.therafoundation.org/ see map;just beyond cablecar. fab pix of dig & its famed murals, PLUS marvy hiiigh caldera view. Grab a gyro in town square, head for cable car! Zip-- visit over!!
MYKONOS-- only time to wander the town's mazey agora and look at jewelry stores, DKNY, Cartier, etc.
RHODES -- Ship right by Old Town.. u can do lightning swoop thru O.T, street of Knights, Grandmaster palace & if u want, Grab bus near Quay for Lindos.. Ruins atop a high cliff. Scenic! Neither is all that authentic; over-restored by Mussolini's Fascists -- but Old Town WALLS & major buildings are awesome!! U can't "restore" them; they are real, from days of the 15th C knights. On Tues & Sats they used to allow walking on top of wall . A line@2:45 pm at G'master Palace -- if still available, a don't-miss!!
PS:Ignore Easy-Cruise recommends. They are cancelling tours left & right, may be bankrupt by now.