West of Athens, the Peloponnisos region is wonderful. Look at Nafplio, Monemvasia Kalamata, Kyparissia, Kardamili,Pylos Koroni, Kyllini (mix of beaches, ancient sites etc, including Ancient Olympia. From the Peloponnese you can reach easily the Ionian islands eg: Zakynthos, Kefalonia, Corfu if you want some island time - also Lefkada which is an island technically but connected to the mainland so you can drive there
Next door to the Peloponnese in the northwest,you have the Epirus region - Parga and Preveza are lovely too. I think Preveza has an airport.
heading inland the area around Ioannina also has some wonderful Lake scenery and as you head inland from there, you’ve got the Pindus mountain, national parks galore etc.
Up in the North don’t overlook Halkidiki and Vourvourou close to Thessaloniki.
Coming down the other side into Thessaly,
There is the Pelion peninsula and Volos to look into.
Finally, directly east of Attica and Athens is Evia - another island connected to the mainland with a bridge so you can drive. Some of the most beautiful beaches. From this side of things it’s an easy boat trip to Skiros,Skopelos and Alonnisos islands.
islands which I do think are worth a mention and maybe you could do some hopping:
Cyclades
-Syros for the neoclassical architecture of the main city Ermoupoli
-Naxos for some of the most beautiful beaches
-Andros, for its uniqueness in the Cyclades in having the green mountains and valleys too.
Small Cyclades: Amorgos for its picturesque Chora and easily accessible beaches. Folegandros for some beautiful Rocky beaches with almost lunar landscapes
In July these will be heaving, so you have a better chance of finding some less bustling spots on Naxos and Andros which are bigger.
Other islands I liked very much and would personally recommend in no geographical order: Ikaria, Kea, Skiros, Ithaca, Kythira