Iceland in high season: What to prebook? [closed]

We will be traveling in Iceland around the complete island for roughly three weeks.

What excursion and sights should one prebook because they tend to be full if booked spontaneously on the day? We have already prebooked the hotels, so that is not my question.

I found at least the Blue Lagoon needs to be prebooked.

How about whale-watching from Húsavík? Is that something you can do spontaneously?