Athens is just about the antithesis of what you're looking for! It's a decent culture and food destination, but nature-wise it's not good. Back in the early 1990s, with non-catalytic cars and before they got the Metro and nat-gas network, air pollution was awful...That's been sorted now, but there are no great beaches within a couple of hours from Athens.
If you want great beaches and don't want to go to an island, I would look into Chalkidiki (near Thessalonika, with the added advantage that you can fly there direct from PRG and rent a car without having to mess about with public transport or the convoluted road network in and around Athens) or Western/Southwestern Peloponnese. If you do want to go to an island, there are plenty of options, but I haven't got a lot of first-hand experience of the islands to recommend anything specific.
The only serious national resort chain is Grecotel, but, of course, there are decent hotels and rooms-to-rent pretty much everywhere that's near a beach!