If you haven't got a car then rent one for a day and head across the border into Donegal. Right across the border you have the start of the Inishowen peninsula. Look out for the Inishowen 100 signs. They lead you all along the coast, passing beauty spots with great views. It takes you up through Buncrana along Lough Swilly, all the way up to Malin Head the most northern point of Ireland, and down the west side of the peninsula with beautiful colours in the sea on sunny days. Take your time, it can easily turn into a day trip.
Click the link below for Part 1 of my Inishowen travelogue for some pictures.