Any field really.
Family Medicine/IM Outpatient (plus specialties): You work a the most 9-5 schedule and once you get in a groove after a year or two, you can slowly start transitioning your efforts into a business.
Emergency Medicine: Offers a great deal of time at quiet hours others are sleeping. Also, a great salary to hedge the risks of investing in your entrepreneurial efforts and being relatively unsuccessful. Think Mattu, the master of the EKG.
Internal Medicine/CCM: Offers a week on, week off so you can spend a good deal of uninterrupted time coming up with a product. Think Dustyn Williams of OME.
Medicine-Procedural: Cardiology/GI can have some equipment/procedural developments, but that's rarer. I'd anticipate that most entrepreneurs in these areas find a way to make time given that after a while their schedules stabilize and they make partner in their practice. Boards/Beyond's lecturer is a Cardiologist I believe.
Surgical: Also can be a challenge, but depending on how you tailor your practice you can find some time off.
Surgical Subspecialty: Others have alluded to device/procedure innovation, but also gain more control over your schedule with seniority and can invest time down the road. Atul Gawande was a surgeon and still managed to become a prolific author. Opthalmologists/Dermatologists who do procedures and see patients and maybe work part time and earn a good amount of money could also do a good job running a legit business unrelated to medicine. I saw a Shark Tank episode with a woman who was an Opthalmologist who made some hair curling device and only worked a few days a week (her husband helped with the business and was a Cardiologist).