Certainly, assuming your flight is relatively on time, you get throught the airport quickly, forego a lounge visit upon arrival, are checked in with a boarding pass for the onward flight, and have a minimal sense of direction and ability to read a transportation map. Take the express train to Termini and the Metro Line B two stops from there to the Colosseum. Tour the Colosseum and the adjacent Forum if you have leftover time. St. Peter's and the Vatican would be too ambitious for your timetable, in my opinion. But I wouldn't pass on the opportunity to take in a small sliver of the wonders of Rome on a layover like yours, unless you are a regular visitor to the Eternal City.

Who needs a pool when you have lovely Nissi Beach across the street?

I mean, seriously. I've never understood people who travel to beautiful beach resorts and spend even a minute of time at the pool. But maybe that's just me.