I visited the Giant's Causeway as a daytrip by train and bus from (London)derry via Coleraine. You arrive at a huge new visitor centre, and then you can set off to explore the formations on your own or with a guide (quite useful for the background info on the history, nature and geology). I was lucky to visit on a warm and sunny September day, and really enjoyed walking along the shore, admiring the rock formations and tidepools, and of course climbing on some of the rocks. Derry is quite an interesting and historic city, and the train ride along the Antrim coast is really beautiful, plus you can see lots of birds of all types. When you happen to enjoy good weather, this is definitely a fantastic site.