Santorini, Kefalonia and Zante are good choices.
The only problem is tha Santorini is in the Aegean Sea and Kefalonia/Zante in the Ionian so it will be a long trip.
Pelion is just excelent. I love this place. Great hikings in from sea to lush mountains.
Then, I do not know what you mean by active.
If you want to try some extreme sports, you can go in Naxos, Paros (cyclades) or Lefkada (Ionian) for kitesurfing. Or maybe rock climbing in Kalymnos...
