All great ideas.

A few things come to mind:

The US dollar is no longer sought after in most countries and, rather than getting a "black market" bargain when trying to pay with greenbacks, you will receive an exchange haircut.

In some African countries, the exchange rate that locals get for $1 bills is terrible compared to $5 bills, so they will quote prices like 3 for $5. If you only want one of the item, simply offer to pay with a $5 bill and take your change in "worthless" $1 bills.

If taking a cruise through a US tourist route, say an Amazon cruise, at the end port, at the end of the day, offer to spend your remaining foreign currency buying back US dollars from vendors who have been fleecing Americans all day.

Americans are "hard-wired" to tip venders and service workers. Do everyone a favor and stick to local customs. Especially important is, if you negotiate a flat fee with a taxi, guide or other entrepreneur, unless they provide more than what you bargained for, the fee does not require an additional tip. Unnecessary tipping can set up predatory environments affecting future tourists. (An example that comes to mind is, if you walk out of the port entrance in Mumbai and take a taxi, it will cost 10 times what you will pay if you walk a block, flag down a cab and said authoritatively "put it on the meter" or simply call for an Uber).

Not meaning to promote, but simply pointing out that Amazon will let you read the first 100 pages or so for free of "Take the High Road - A Primer for the Independent Traveler" (amazon.com: Amazon.com: TAKE THE HIGH ROAD) by clicking on "Look Inside" which should supply, well, the first 100 pages of ideas :-)

While, admittedly, we sometimes spend a pile on travel (and frequently not), we always try to figure out ways to squeeze the best value out of what we get at the same time as maximizing the experience.

Jeff