The question would be 'How was the first day in the studio?' if you used 'how' instead of 'what'. Either are acceptable, although to me the version in your original question sounds more natural.

I think you could put: "How was the first day in the studio?" In fact it sounds more naturally. I would use "What+like" whenever i refer to a person or an object. For example, "What is your son like?" "What is your car like?" But thats just my opinion. Please correct me if im wrong. :)