I visited Iceland last year around the first week of September but only did day hikes and regular tourist stuff. I recall the travel agent telling us how lucky we were that we only had one day of unpleasant weather (high winds and rain). If you are going inland, and I assume you probably will be, be prepared to for cold, wet weather. As in it-might-snow cold. The coast is a bit more temperate.

Besides weather the only other thing I can tell you is that Iceland is a very expensive place to visit. Just about everything is imported and the prices reflect that. That said, you'll just about have to try to not like Iceland.

It's a beautiful country and I wish you a great trip!