I'm assuming you arent a nurse yet, am I correct? I added RN schooling into each of my responses below since that is where you will need to start..
A MD obviously makes the most, but they also sacrifice most of their time to their program, residencies, and studying. Its 4 years pre med, then med school, then residency, then practicing on their own.
As for a CRNA, realize that its a full time program. You would also need to get your RN first, then return to school for 2 years to become a CRNA. You will most likely need 1-2 years experience in an ICU type setting. Yes, they make good money, but its also very time consuming.
For your MSN, you'll have to have your RN followed by 1-2 years experience, followed by 2+ years of schooling (full time). This Route also allows you to become a nurse practitioner, which has flexible hours as boomer has stated. Its still nursing, but advanced practice. You have more autonomy and work with a Dr in this position such as in a Dr's office, clinic, or in the hospital setting.
A PHD in nursing will mostly allow you to teach, or assume a higher role as a nurse in a hospital/nursing home/rehab center such as Director, or Dean of nursing.
Good luck in whatever you choose!