Just went for a couple days for the first time in October. I loved it and want to go back. Renting a car in the city was pretty affordable and definitely worth it over some bus trip, particularly for the northern lights. Thingvellir was cool. The Golden Circle route takes you to Thingvellir, Gullfoss, Kerith, and Geysir. If you've already been, I'm sure you've done this...

Hope it isn't cloudy like it was for us, and hope there's a big solar flare on Sunday so you can see some activity