Activities pre and post RS Athens tour

No, the Greek islands are not all off limits. One island, my least favorite one,Santorini, is dealing with possible earthquakes. Look at the other hundreds of islands. Or spend extra time on the Peloponnesean peninsula.
How many extra days you spend depends entirely on you. Rick Steves offers different tours of Athens so I don’t know how many extra days you’ll need.
I would add a minimum of four days to the beginning and end of the tour. Does your tour go to Thessaloniki? Visit the islands of Paros and Naxos. Stay in Napflion for a few more days.

It looks as if the earthquake swarm is only in a small section of the Aegean Sea near Santorini. That shouldn’t affect travel to other islands.

Hi there, I would suggest you start here, if you haven't read the RS overview of parts of Greece - https://www.ricksteves.com/europe/greece. What are your interests? If you say, 'Ancient Greece' well, there's so much to see. Agree with above poster, arrive 4 days early to be sure you get there even with travel delays before the tour starts and that gives you time to get over jet lag. Personally, I'd head to the library, where I go even when I've already been someplace, to refresh and broaden my horizons. DK guides have pictures and great overviews of sites, which I find helpful.

Using the tool bar above, I searched 'Greek Trip Report', here's a variety of great reports from other travelers. https://search.ricksteves.com/?utf8=%E2%9C%93&query=Greece+Trip+Report&button=

Good luck!

Thank you everyone. You have given me some ideas, I’ll keep on reading.

Try to arrange your plans so that you don't need to catch a ferry back to Athens on the day before your flight home. There are occasional cancellations due to weather or strikes.