I did a small day tour with an outfit called GeoIceland Tours.
I found the Golden Circle Tour to be really nice. Small bus. Fun people. Great guide.
That being said, I had a quick 2 night stay. I could have done a 3rd day, but that would be about it.
I found Iceland fascinating, but too stark and cold for my tastes, and I was there in June. I am glad I went, I have no compelling desire to return.
I went on to Germany and had a blast there.