There are lots of things you can do with your commercial pilot certificate and a second-class medical. The FAA asks you about them on the Knowledge Tests and the oral portion of the Practical Test. Many of the test prep and oral exam guides mislead students into thinking that 14 CFR 119 lists all of the things you can do. But careful reading of the FAR indicates that these are things you can do with your own airplane and not have to comply with part 121, 125, or 135. Note that it does not limit what you can do if you are not using your own airplane or if you are working for an operation that is complying with part 121, 125, or 135. There may be other regulations that you need to comply with e.g. Part 137—Agricultural Aircraft Operations has rules for aerial spraying and sightseeing flights may require a letter of authorization (LOA) from the local FSDO.
§61.133 just says that you may now carry persons or property for compensation or hire but doesn’t go into detail about what that means.
(a) Privileges—(1) General. A person who holds a commercial pilot certificate may act as pilot in command of an aircraft—
(i) Carrying persons or property for compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation; and
(ii) For compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation.
(a) This part applies to each person operating or intending to operate civil aircraft—…
(b) This part prescribes—…
c) Persons subject to this part must comply with the other requirements of this chapter…
(d) This part does not govern operations conducted under part 91, subpart K…
(e) Except for operations when common carriage is not involved conducted with airplanes having a passenger-seat configuration of 20 seats or more, excluding any required crewmember seat, or a payload capacity of 6,000 pounds or more, this part does not apply to—
(1) Student instruction;
(2) Nonstop Commercial Air Tours…
(3) Ferry or training flights;
(4) Aerial work operations, including—
(i) Crop dusting, seeding, spraying, and bird chasing;
(ii) Banner towing;
(iii) Aerial photography or survey;
(iv) Fire fighting;
(v) Helicopter operations in construction or repair work …
(vi) Powerline or pipeline patrol;
(5) Sightseeing flights conducted in hot air balloons;
(6) Nonstop flights conducted… conducting intentional parachute operations.
(7) Helicopter flights conducted within a 25 statute mile radius of the airport of takeoff…
(8) Operations conducted under part 133 of this chapter [rotorcraft external-load operations]
or 375 of this title [NAVIGATION OF FOREIGN CIVIL AIRCRAFT WITHIN THE UNITED STATES];
(9) Emergency mail service conducted under 49 U.S.C. 41906;
(10) Operations conducted under the provisions of §91.321
[Carriage of candidates in elections.] of this chapter;
(11) Small UAS operations conducted under part 107 of this chapter.
In addition to all of these things you may also (for compensation) provide pilot services to someone who owns their own airplane and operates it under Part 91. You may be part of a corporate flight department. You may pilot aircraft for companies that provide air transportation as an incidental part of their business. e.g. wilderness adventure companies where you transport clients to remote sites, real estate companies where you transport clients to visit locations.
As another example, if someone without an instrument rating needs to depart you airport in IMC conditions, you can fly them to VMC conditions and take a taxi back for compensation.
The FAA does not want you to be acting as a charter operation without complying with Part 121 or 135. AC No: 120-12A goes into detail about this.
There is no clear definition of “holding out” but there are some bright-line examples. Putting an ad on Craigslist is definitely “holding out”. Flying someone you know to their remote locations in their plane is fine. Flying their friends plane is fine too. Being generally known as someone who is willing to fly anyone at any time is crossing the line.
Letting the FBO know that you are available to fly people over the hill when the airport is fogged in is probably fine. Putting the words “Commercial Pilot” on your resume or website is fine.
Having a non-pilot rent a plane and then pay you to fly them somewhere is not fine.
“Holding out” can be as complex as publishing a flight schedule for a major airline or as simple as posting a notice on an FBO bulletin board (or the Internet) telling everyone you’re the one who will fly them to that prime vacation resort and make their dreams come true.
There have been several attempts to establish an Uber/Lyft type operation for aircraft. The FAA does not approve of any of them—even if you have a commercial certificate.
If you are going to fly for compensation and it is not explicitly permitted by the regulations, it would be wise to seek the advice of an aviation attorney.