Apparently not very good for mountain biking – the interior is just proper exposed & windy – a lack of trees offers no shelter.
I went in May 2007 the evenings were starting to draw out nicely, barely saw any darkness. I expect April will be too late for the northern lights.
As others have said the golden circle tour is good. The iceberg lagoon at jokulsarlon is truely a magical place. We hired a car & explored the south side of the island, every time we went round a corner we were treated to another fantastic view. Definitely want to go again one day, but there are plenty of other places I want to visit first!
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Icebergs at Jokulsarlon[/url] by zachwinsley[/url], on Flickr