We lived in Iceland for 2.5 years. (1988 - 1990) It's a beautiful country, with friendly citizens who speak passable to excellent English. You've chosen the best time to visit: August & September. The temperature is warmest then. The little pink & purple flowers are blooming in the moss that covers the lava fields. The waterfall are magnificent, as the glaciers have been melting all summer. Long hours of sunshine. The country is small and you can get to almost anywhere in a day or less. Too many specific things for me to recommend seeing, but I'd put Thingvellur near the top of my list. Enjoy yourself!