A full senior-level interview round, live coding or mobile system design, your choice, conducted exactly the way I've run them as a hiring manager. Then the part candidates never get: 15 minutes of honest debrief, what I'd write in the hiring packet, hire or no-hire, and precisely why. You leave knowing how you actually perform under real conditions, not how you hope you do. Based on the system in my book Share Your Screen.
I take a limited number of sessions per month. Most of what I know is in the books at salari.dev.