I haven't stayed in Nidri but visted a couple of times. There are plenty of restaurants and bars overlooking the sea and off the main road. If you hire a car, there are lots of great beaches off the west coast and little bays in the south, all well worth a visit and you can also get up into the mountains. There are plenty of boat trips from the harbour at Nidri, round the island or to neighbouring islands and of course the ferry goes acrosss to Meganissi where we spent a week. One boat trip I would highly recommend is the "sunset and stars" cruise. It sets off from Nidri just before sunset and heads for a beach on the mainland that is only accessible by boat. While the sun sets, you have a buffet meal on the beach until it is dark. Then the guide, who is a real character, starts his stories and guide to the stars, pointing out the constellations and telling the Greek myths associated with them. He is a great storyteller and it was really fascinating. Away from the street lights you have a gret view of the stars and we saw lots of shooting stars and even the international space station. A very enjoyable evening, highly recommended!
Some pictures from our trip to Lefkas:
http://www.worldisround.com/articles/348861/index.html
I'll include our Meganissi trip, as this has a couple of pictures from the "sunset and stars" cruise:
http://www.worldisround.com/articles/337377/index.html