Put a simple sign up that says “Will teach you X” while driving. This could be Singing or another language (Arabic, Spanish, Polish or whatever languages you speak).
Just say that you will do this for pure tip or add a price – e.g. $10 per 10 minutes.
Being in the cab is a sunk cost in terms of time. You have a captive customer. Do not hassle them with your sales pitch though. The sign should be enough. The customer wins as they learn something while time could have been wasted. You win as you increase your income while also enjoying teaching something that you love.
The teaching does not show up on the meter either so you get to keep 100 % of what you earn as a tip.