you’re going at the best time! I’ll add to the list:
if you go to Jökulsárlón (which is spectacular even if you don’t go out in the boat – the icebergs flip and make sounds, eider ducks cruise with their chicks in tow, and you can walk out on the black sand beach across the road and see icebergs parked on their way to the sea) - right next to it along the ring road (east) is a smaller one, Fjallsárlón, which I liked better, probably bc at that time, there were fewer tourists surrounding it. also in the south, friends of mine recently said Vestmannaeyjar was the highlight of their trip. I haven’t been yet – it’s a small island off the coast, I think there’s a ferry.
for birds, I loved Lake Myvatn in the east, plus the formations around the lake looked like the Star Trek planet. so many birds everywhere, and every viewpoint was more dramatic than the rest. at the lake, wearing a bug net to go over your hat is a good thing – they also sell them in the convenience store at the bottom (south) of the lake.
you’ll be there during arctic tern nesting season, I saw them nesting near the Reykjanes lighthouse, they dance in the air and make so much noise, but you can get pretty close to the area and get great views of them. don’t walk underneath though bc they lay their eggs right on the ground, it freaks them out if you get near the nest, also it would be easy to step on an egg. but you can get to the edge of the flock. huge geysirs there too as the plates are moving apart. also, closer to town, the terns nest near the golf course at the end of Seltjarnarnes peninsula, which is just east of Reykjavik, just bike or walk along the shore path (paved) to the tip and you’ll get to the bird reserve! you’ll also pass Thufa (in old harbor in reykjavik) and the Kvika footbath, same artist, very cool. I can also recommend Floi bird reserve in the south of iceland, marshy, saw some red-throated loons there.
also in reykjavik this last time I met some very cool people at “The Space”, among other things they run photo expeditions to the Westfjords, Greenland and other places on this beautiful wooden ship.
oh in terms of the Golden Circle, it might be a fun option to go at midnight (the sun will still be more or less up – I’ve been in day and night, and the thing about the night visits is that they’re open (not the gift shops, but the areas) and the swarms of tourists are gone. geysirs in the midnight sun are something else – you can also do some field recording (which is what we did) without the crowds! also, Thingvellir looks like something out of game of thrones, I loved exploring it.
you’ll have so much fun!!