Heimdall

I'm not really sure what your idea of "touristy" is. If you mean busy, none of the islands, even Mykonos and Santorini, will be busy in mid-May. If you mean souvenir shops, etc, then any island that depends on tourism will have them.

Folegandros is beautiful, and very popular with walkers. I was there at the end of May, and as clausar wrote it was very quiet. Mid-May will be even quieter, especially outside of chora, which is the only settlement of any size on the island. Some people love Folegandros, but I was bored after the first day.

Tinos has more Greek than foreign visitors, so perhaps that is what you are looking for. Syros also has fewer foreign tourists, and is the administrative capital of the Cyclades, so if you want to see Greeks going about their daily lives maybe that would be a good choice.

I don't often mention my favourite island, Antiparos, but that is another to consider. Antiparos is the island Tom Hanks and Rita Wilson chose for their holiday home. It's quiet in mid-May, but only 7 minutes by ferry from Paros. If you have a car, you could stay at the beautiful southern end of the island: http://www.dolphinantiparos.gr/.

KayF

We've visited Naxos and Folegandros on separate trips to Greece and if we had to recommend only one, it would be Folegandros. It had a wonderful, laidback feel and the tiny main town is so atmospheric and full of charm. We didn't do much on Folegandros as it was August and stinking hot but we loved where we stayed and loved the 5 min wander into town for coffee or a meal and to soak up the ambience.

Kay