It's a few years since we visited Nidri, Lefkas but here goes:

1. Boat trips. Several. We went to Ithaca, Kefalonia, Meganisi and even Skorpios (don't even think about landing there because of the Onassis armed guards but you may swim. Do NOT leave your passport on Meganissi before a 3 day storm with only 3 days before you come home (stupid boy).
2. Waterfalls. A lovely waterfall about a half mile walk from Nidri. You may walk up a very narrow rock path beside the waterfall. I must lose some girth before I try it next time.
3. We stayed in the Three Brothers pension, right on the beach. Breakfast in their taverna downstairs. Comfortable, clean and balconies overlooking the sea. I am not sure whether it's still there but I hope so. Plenty of fireflies and frogs in the river bed next door for intelligent conversations.
We split the holiday with a week in Parga which worked well.
I expect a more recent visitor will be able to update this.

the waterfalls just in from nidri are the rachi falls. set is cool green ravine. the falls are pretty but you will also find numerous lovely picnic spots by the river as it falls down towards nidri. really fab if you want a bit of respite from the heat of the summer. so peaceful and cool and green!
http://www.trekearth.com/gallery/Europe/Greece/photo487991.htm

there are numerous boat trips from nidri. there are trips around the outer satelite islets of lefkas such as skorpios, onassis island and meganisi.

i think there are trips round to egremnoi and porto katsiki beaches which are amoung europe's most stunning but quite difficult to reach by land (especially the former).

you can also get the captain aristethes to fiscardo on kefalonia and frikes on ithaca. the boat leaves early morning and comes back mid afternoon so you dont get long there unless you stay overnight but it is a nice boat trip and a great way to see the region!

Can anyone confirm whether you are allowed to take a hire car by ferry from Lefkas to Ithaca please?

We are hoping to split a fortnight between the two and take in Meganisi as well, but I don't think we'll need the car for it, just a day trip from Nidri will cover than I think.

And while I am here, does anyone recommend a jeep rather than an ordinary car? I mean are the B roads in a terrible state??