I've done both proving runs for a new certificate and "validation" flights for the addition of another type to that certificate.
We carried our POI, PMI, PAI, FAA "resource" on the type (their expert), as well as a POI-in-training. Basically, we had to show how we knew we had an airworthy aircraft, could follow 135, and handle unexpected events. Every scenario was delivered on a post-it note. Between legs, they would introduce a mx problem to ensure that we knew the steps to have items fixed when away from base. We also did approaches at every airport. They didn't mess with us too much during the flight, and the most stressful part was the mx hoops, made worse by our long-winded company procedures, combined with real mx issues.
Feel free to PM me with any specific questions you have.
It goes without saying, but know your Company Manual, OpSpecs, and 135...and be able to apply it to the flights. And do not rush through things...always consult the book!