To echo what AutoSlash said: When I visited several cities in the Charlotte area, I had a mix of free days with Hertz and National. I used them for the most expensive one-ways, from areas where you can either choose your car (National) or book a premium group G directly (Hertz). I found cheap one-ways with other companies to fill in the gaps.

Some cities don't require a car if you stay downtown, so you can break the trip that way. Note that if you do this, however, sometimes the weekly one-way rate is a much better deal than the daily.

It's annoying and inconvenient to switch cars and companies along the way, but it can result in substantial savings.