With only five nights, extreme efficiency in transport is vital. And more information is needed. What TIME on June 8 can they leave Barcelona?? and -- since u seem to indicate that the end of this adventure will be the morning of Friday June 13 -- where do they have to be on June 13, in order to fly back to US? Come on Mom, share; we know lots of stuff but are not mind-readers! If they are due to leave for US from ATH airport, they have several options. First, how to get to ATH quickly/cheaply? If they can leave at 12:01 AM on June 8 they should JUMP at an Aegean Airlines flight currently offered on Mobissimus Website for $55 to ATH, arriving 4 am (they can drink espresso, then nap later!!!). IF this is feasible, several options include:
(1) NAXOS - X 96 bus (€5) from airport curbside to port of Pireaus in about 70 minutes; 7:25 am Blue Star ferry @€35 (large, modern, open decks, capac. 1500+ passengers) ARR Naxos about 12:45. June 8-9-10-11- to 6pm 12th, enjoy Naxos gorgeous beaches, water sports, hiking etc. Can find "studio" units sleep 3, on/near beaches for 35-50€. On Th 6/12, check out@noon, stow bags at hotel, have last aft on beach, take 6 PM Blue Star, arr Pireaus Midnight. Taxi to central ATH or bus direct to Airport, depending. Here's a great photo-album by "stanbr" of Naxian delights: http://www.flickr.com/photos/stanbr54/sets/72157634605629689/
(2) NAFPLIO -- since too young to rent car/van, X93 bus to Kissifou Intercity (KTEL) bus station; Comfy reserve-seat bus (€13) goes hourly; a scenic drive (2.5 hrs) along sea, over Corinth Canal, to Nafplio, voted the most beautiful "Old Town" in all Greece. Its on a peninsula, feels like an island, sunsets from harbor are sublime. Check great noncommercial website of Nafplio & area highlights: http://www.visitnafplio.com/visitnafplio.com/Home.html The 6 can stay near TOLO (Tolon) Beach 5 miles outside town. Because Tolo itself is mainly biggie hotels, I suggest a modest place just across from Tolo beach, on small "Assini" cove -- http://bungalows-villa-christina-assini.focusgreece.gr/en/aboutus.htm -- hourly local bus (15 mins) to Nafplio for sightseeing. Swims, boating, hikes. Feels like an island, with convenience of mainland... and easy/quick to return to ATH day or evening.
HOWEVER -- since u omit vital info like lodging budget, and Where they fly back to US, it is difficult to give advice. Do not worry about a lengthy explanation; the more facts the better! It keeps us from wasting time/effort on inappropriate scenarios.