Are you hiring a car? The Golden Circle is definitely worth a day of your time either driving or by coach.
We’ve spent about 3 weeks doing various bits of Iceland over 2 holidays. The most fun has been when we’ve been in the really remote bits and felt totally removed from civilisation. E.g. did a 6 mile walk around one of the lesser-trodden glaciers and didn’t see a soul the entire time.
Oh, do a walk on a glacier too if you can.