There's nothing in Thermopylae but a monument, and the terrain has altered so radically in 2400 years that you won't be able to visualize the battle layout. In a visit of only 5 days, you don't have time to go to Meteora -- you can't pour a quart into a pint bottle, no matter how you tilt it. I guess I qualify as a seasoned Greek traveler with 13 trips, 5 of them with newbies like you along. So here are my thoughts.
What you should do first depends on where you're coming from, and what time of day u arrive in Athens -- neither fact provided. IF you have a nonstop from N. America, arrive before noon and aren't too exhausted, I would suggest you rent a car & go to your first destination upon arrival. You could drive to DELPHI, get a hotel, check out the wonderful museum, then crash for a couple of hours, then enjoy a sunset dinner. Next AM, (Day 1) get up early go to the ruins by 8 am & you'll beat the bus tour crowds.
Leave by 10 or 11, and drive to NAFPLIO... Use GPR for directions or michelin/plann my trip. Also here's a good online map http://euro-map.com/karty-grecii/peloponnes/podrobnaya-turisticheskaya-karta-peloponnesa.jpg On the way you can stop at ANCIENT NEMEA, just about 5 miles off the big intercity hiway.. It's a sacred-games site, runner-up to Olympia, authentically excavated.. Temple columns, and a swell stadium- your lad can race from starting-block 2300 yrs old. then on to NAFPLIO. Greeks consider this the most beautiful Old Town in the nation... it is on a peninsula surrounded by water, and is Ground Zero for Ancient stuff - stay there 2 nights; here's a guide to the highlights, read every link http://www.visitnafplio.com/
On the Morning of Day #3, on your way out, drive up the back road of PALAMIDI (the gibraltar size Rock looming over the town, with a fortress on top), take fabulous photos. Drive EAST to EPIDAURUS -- don't spend much time on the ruins, its the world-famous Theatre that will amaze you... you don't have to backtrack, just go up the Seafront road to ISTHMIA, and jump back onto the big intercity --- go back to Airport, NOT into Athens, return your car and get a TAXI (flat rate €38 to Central Athens) to a hotel, That leaves you 2 full days -- #4 & #5 -- to see Athens landmarks.
As for getting the "6-pack" ticket, it's up to you... just €30 Euros vs €20 for Acropolis only. Some of the other 5 y ou can do as "walk-by" -- Temple of Zeus and Roman Agora, for instance -- but you probably will want to explore the Ancient Agora. If you are not well-versed on Ancient stuff, Rick Steves has provided wonderful podcasts on this website you can download FREEEE ... to do step-by-step DIY visit to Acropolis, another to Ancient Agora, another to the "top 10 things to see" at Nat Arch. museum.
