Renting in one city and dropping off in another.
One-way rentals almost always carry a drop-off fee, which can vary enormously depending on the distance and whether the return location is in a different country. It's worth comparing this fee against the cost of round-tripping back to your pickup city before assuming one-way is more convenient.
Cross-border one-way rentals are sometimes restricted entirely, so it's worth confirming with the rental company directly rather than assuming a route is permitted.