I'll level with you here. I am also one of those high IQ people. I am, objectively, super smart. I could not work in a field that did not stretch me out of my intellectual comfort zone on a daily basis. But raw cognitive prowess is not enough to qualify you for any particular specialty, nor does it speak to what type of nursing would be most intellectually/professionally fulfilling to you. Personally, critical care would not scratch that itch for me. I don't like bedside nursing, and definitely don't like having to constantly monitor drips and VS parameters and all that. SO MUCH CHARTING!
You can find endless opportunity to flex your cognitive muscles, no matter the specialty. Just depends on how much of yourself you are willing to devote to it. Find a job/career path that excites you, and if you follow it ruthlessly enough you will find yourself challenged to the limits of your intelligence quotient. If you love the elderly but are worried you won't be happy as a floor RN at a SNF, then become a case manager or a geriatric NP, or specialize in hospice, etc. For example.
Anyway. Midwifery is obviously the right answer here. Most challenging and rewarding specialty there is. I moonlight inpatient as a triage midwife, and work full time for Planned Parenthood. I've worked in reproductive health care my entire adult life, and the more I learn, the more I realize HOW MUCH MORE there is to know! Plus, my work has real and immediate impact on the health of my community, and I get to help people in a profound way, on a daily basis. So you should definitely do what I do