Hi Peter,
heaps to do and see.
I was there at midsommar so didn't see the night. and was actually in europe visiting friends and decided last minute to go to Iceland, which resulted in the need to pick up a cheapie 'point and shoot' camera at the airport. didn't go home with no pics at all. i would love to go back and do the photography properly.
but yes there are great geographical features everywhere. the althing area was quite amazing - it happens to be the boundary of the n american and euro plates. i think you can go swimming in the water (massive wetsuit required) and its supposed to be super clear.
i really liked bjarnfoss (i think it was called - maybe it was because the sky was blue then

)
A car is a must to make the most of the scenary, i was there on a shoestring and took the bus tours.
One tip is if you're strapped for time do blue lagoon on the way to the airport (flying out), a good way to loosen the muscles before a long flight.
Rusty