need suggestions for a two to three day extension of my tour

If the tour does not include Athens and you have not been there, that is one idea.

Nafplio would be a good ' hang out and enjoy' place

My tour starts in Athens. I'll have about 3 days there.

My days up at the Meteora monasteries were fabulous. There are bus tours that go up there and private tours, depending on your budget. I believe you can also do it by train. We did a private driver up there because I wanted to see each monastery on my own schedule. They are gorgeous. It was a nice contrast to the rest of our Greece trip - very different from the islands, Athens and Delphi.

Here's a little overview with good pictures: https://www.greecetravel.com/meteora/monasteries.html

You are getting some good suggestions. There is an overnight bus tour that does both Delphi and Meteora which will give you two world class sites.
Delphi and Meteora https://www.flickr.com/photos/stanbr54/albums/72157698930270992

If you are fed up with touring Nafplio is a really good option. Find a hotel in the old town. Our favourite is Agamemnon. Its the only hotel with direct sea views. If you still feel the urge to tour there are local buses which take you to places like Epadavros or perhaps you can find a way to get to the Byzantine city of Mystras at Sparti. Thats a bit of a long day though.
Nafplio https://www.flickr.com/photos/stanbr54/albums/72157632094108982

thanks guys. I think I'm going to do the Motera monastery tour and a day three island boat trip. Should be fun.

Meteora fits your bill perfectly.

For hotel, we recommend Doupiani House, right under the rocks in the quiet village of Kastraki. https://www.doupianihouse.gr/

For tours of the monasteries and village, check out Visit Meteora. https://www.visitmeteora.travel/

I suggest a re-think 3-island tour -- really a long sea-trip, with short port stops. Not as bad as high season but still crowded. Nearly 3 hours to reach first isle. Read the reviews - https://www.tripadvisor.co.nz/Attraction_Review-g189400-d2716449-Reviews-or5-Hydraiki_Athens_One_Day_Cruise-Athens_Attica.html -- Biggest complaint: not enough time each stop -- Hydra 2 hours, Aegina 3 or so, Poros 45 mins. NOT relaxing, SOME good reviews, but for off season -- either October or early Spring.

You may only feel comfortable on a tour. However, if DIY is OK -- go to ONE island, Aegina. See this fab June 2009 photo album of a D-I-Y day-trip to Aegina - http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ I've given Aegina hints so often -- I'll just "cut & paste" my advice, adapted to your situation:

• STRONGLLY favor weekday - Friday Sept 10, or Mon-Tues 13-14 -- Athenians crowd it up as a weekend getaway.

• Take the GREEN LINE METRO to Piraeus (if you can walk to Monastiraki station, then it's direct, no changes; 30 ;mins to port). Be aware of pickpockets; not nearly as common as in other European cities but they DO target that Metro line.... because of a lot of gullible N. Americans who never do public transit!! Just take money for the day, leave passport etc locked in your suitcase in hotel.

•At port, just buy a ticket at any agency on street. For at least one-way, take a CONVENTIONAL ferry, NOT a hydrofoil (ormer have open decks where u can sit , latter are enclosed, spray-coated windows, NO view). The sail is part of the xperience - a "seascape" of huge cruise ships, ferries large & small, motorboats, day-sailers. Hydrofoils r hourly Regular ferries less frequent -- 1st is 8:20am, then not till noon. If you can't rise early for the 8:20, then have inbound trip be your "deck time". Last of these is 6:15pm --so you could have sunset on board. No need to buy round-trip, but upon arrival at Aegina, buy inbound ticket, just for convenience.

• PORT SIGHTS - You could do some on arrival (museum may close at 3), and the rest in late day. Right by ferry is a temple ruin (ONE column left! I got a photo against column, framing a sea view... it's on my wall!). TINY museum next to it has AMAZING "Ali Baba-worthy" man-size jars. Back-streets are fascinating, small shops etc. Aegina is "backyard" for Athenians, not "tourist mecca" so back lanes typically greek.

• OUT & ABOUT - Check bus stop timetable for bus to center-island Temple of Aphaia... most complete temple in all the Greek Isles (would be totallly complete if 2 Brit Victorian THIEVES had not stolen all the artworks). It's on hill with stunning 360° view of whole Saronic bay. Takes 15- 20 mins to explore, and next bus won't be for hours. So just walk the road downhill & look for tree-mounted sign "short-cut"... scramble down and presto! you're on a path into seaside village, Ag Marina. Narrow sandy beach (wear swim gear under clothes, change in a taverna restroom, & have a refreshing dip). Then enjoy a lovely lunch al fresco, take bus or taxi back to Port town. There's a famous monastery on the way, but I didn't bother with it. Option #2: - Hire taxi, get "set price" agreement for taking you to Temple. waiting 15 minutes for you to view, then onward to beach of Marathona -- here's a lovely place for lunch, swim, sun http://www.aeginagreece.com/aegina/pages/beaches/la_palma_marathonas_cafe_bar_restaurant.html U probably can get bus back to Port town.

• LATE-DAY - Wander the town, then a late-day drink at portside cafe, then board conventional "deck ferry" for sunset hour back to Piraeus... a relaxing end to a day's getaway. If you don't fancy the Metro. Taxi from Piraeus to Plaka area should be about €20 - ask your hotel for an estimate, & if u are super-cautious, have them write it in Greek to hand to driver. Never hurts.