Was there two months ago. Aside from the fabulous Acropolis and a few other historic sites, Athens is not an attractive city, easily the least attractive major city I’ve been to. There are a lot of good restaurants in the area near the Acropolis. The places others mentioned are worth it. A few days in Athens is plenty, so if you have the time, get to Mycenae, Olympia and Delphi. Getting to the islands in a 5 day stretch including a few days in Athens would pretty much exclude the mainland sites. Too spread out. Santorini is beautiful. Crete is probably the most interesting historically along with Rhodes.
We loved Greece physically beyond the islands, Greece is generally beautifully rugged looking. It is not an easy walking place. Many of the historic places require extended uphill climbs, often up paths and steps that are uneven and worn smooth over a few thousand years.
Bon voyage.