I have a couple buddies who own breweries, including some nationally known ones (spouses run them though). One just has a restaurant that brews that launched statewide in the spring. 2-3% ROI from restaurant, ~16% from bottles. I dont know details of others, but based on lake mansions, they seem to do well. These are MDs with early savings that seeded the business.
Another friend invested in medical marijuana dispensary and related growing. That one is pretty shady to me, but I am sure he will make plenty of money since it is one of just a few in that particular state. I cant bring myself to invest in that stuff. It is being run by spouse of another of his partners.
Some groups own a portion of their hospitals/surgery centers, so I dont know if you count that. They are more involved with the business end of those facilities than the typical group, and apparently they can kick out a very good ROI.
Many are doing "small scale" real estate, 1-30 properties. Come to think of it, I know one guy who owns one of the two sides of main street in his small town, literally. This includes malls, restaurants, grocery stores, drug stores, etc. This was built over a career with money from anesthesia, and then from that endeavor feeding itself. Quiet, humble guy you would never expect it of, nor would you know it unless you looked at the deeds. Drives a camry, modest house.
Owning farm land is a common one in the Midwest, almost nobody does the work themselves, but if you just do veggies all you have to do is plant and they will harvest. Largest guy I know owns 600 acres, farms 800, but he has significant help from brothers.
I dont know of any except the guy owning main street who are the primary business manager and work full time within anesthesia. They all do a fair amount of work, but the brunt of it is being done by their spouses.
Also, none of these people do as well as the partners who have invested wisely in businesses being run by non-MDs, with the exception of the nationally known microbrew guy, but his spouse was a great business woman with her own career before they started that.
Suffice to say that there are more MDs out there than you would expect that have fully funded FU accounts and backup careers, but still slug away in the OR daily.