Recommendations on how to spend 3 extra nights after RS tour

We flew to Naxos after the tour for a few days before going home. The island was bigger from Hydra and a nice way to relax after a busy tour before the long flight home. We had come in two days early before the tour, so with the days spent in Athens during the tour, we were ready to see something else.

We also had 3 extra days after our RS Greece tour ended.
We rented a car and drove to Meteora and spent the time there.
One day to drive to Meteora. The second day to take the day tour and the sunset tour with Visitmeteora.com.
The third day to drive back to Athens Airport where we returned the car there and stayed at the Sofitel before flying home the next day.

If the tour is staying at the Acropolis view or the Hotel Hera, You are one block away from Siggrou Avenue where many of the car rental places have an office. So after our tour ended, we walked and wheeled our luggage from the hotel to the car rental office and picked up the car. Since it was a Sunday, driving out of Athens to the E75 wasn't bad at all.

You have to spend your last night in near Athens before your international flight. So assuming that your three nights has to include your last night before flying out, you only have two extra nights to spend somewhere else.

Personally, I would go to Meteora. Two nights is about the right length of time while in my view it is not enough to make it worth going to Naxos or another island. You could, of course, spend the extra days in Athens and never run out of things to do but Meteora really is special. It is totally different than anything else you will have seen.