Give it to charity, Invest in the stock market, leave in the bank for reserves for whatever, or buy more rentals.

Or extend to 25 or 30 years and have the payments go to his children, or church, or charity, whatever he wants if he passes away before it's paid off.

Freedom, money, time, cars, fancy houses, family, kids, travel, charity, faith, community service, side chicks, boy toys, or the ability to just be lazy.

She declined that offer, but then I offered to buy as much as she could gather, with the understanding that 100% of it would be donated to charity and she happily accepted. 

I have 6 tenants, though one is on financial assistance with a charity (not S8). 

I'd like to get to a point where I can give a substantial portion of my income to charities, and perhaps even create a foundation for the causes I care about - environmental protection, human/animal welfare, social justice, etc.- Buy a piece of land to care for and to create my dream cabin in the woods- Learn from a mentor (through reciprocity of course, giving where I can), be a lifelong student and teacher, and ultimately mentor others who are seeking financial freedom through real estate.I have a LOT of entrepreneurial juice at the moment, like busting at the seems.  

Should I stop giving to those charities and instead let some complete stranger make money off my property?

Don't leave bequests to charities in your will or trust.

You can do that via volunteering with charities or targeting specific groups. for instance, in my area there are a lot of physicians so you can find out what events they attend and see if you can present to them.

The reason is that in the second case your "Charity" is open-ended.