After a week on the mainland, my friend Rheanna and I returned the rental car to the airport and caught a bus to Lavrion. When I read online that you had to take a bus to the town of Markopoulos, I was concerned that we wouldn’t know where to catch the following bus to Lavrion. In actuality, it was quite easy. The bus stop for the KTEL (suburban buses) was outside the airport near the escalator that takes you up to the Metro line to the city. I also asked for help from the tourism booth inside the airport. Once we arrived at Markopoulos, we hung out and waited for the next bus. We weren’t at a bus station, just a corner in town where all the buses stop. The whole trip took under an hour, not the 2-2.5 hours that we were told. Check out this link for the bus schedule. I was surprised to find that the buses in Greece were on time consistently. I can’t say this would be true in high season, but it was in October.
We spent two nights on Kea in an Airbnb.com in the very small town of Otzias about 5.5km from the port town. We knew the buses were no longer running so we grabbed a taxi for 8€ to get there (after buying provisions in town).
In my opinion, Kea was too small to really need a car/pay for a rental car but being there longer without bus service would have been tough. We spent one day hiking the footpaths that, while not well marked, were enjoyable–taking us through pastures, over rock walls and fences, and sometimes into farmers’ backyards. The routes are marked, but we didn’t see any markers until a few hours into the hike. I used Google Maps and the location feature on my smartphone to help us along (thanks T-Mobile global coverage!). We hiked 2.5 hours up to the town of Ioulis, the capital of the island. It had more restaurants and shops then we expected, though a lot of them were closed this late in the year. We grabbed lunch and then headed down to the port town of Korissia. We grabbed an early and cheap dinner in Korissia (location coordinates 37.658647,24.311807). The gyros were 2€ each and the wine was cheap at 3€ for a half carafe.
In summary, I would recommend visiting Kea but before the buses stop running at the end in September. You could easily spend a few days hiking from town to town and swimming in the small bays around the island. Also, this site seemed to have the most accurate ferry schedule including the changes in low season.