In 1988-1995 DL did have a standby airpass for 30 days or 90 days for US visitors. For 500$ you could just show up at the airport and then go wherever you wanted, as long as there was a seat left on the plane 30 min before departure. I did just that and thought that all travel in the US was like that.

It was limited at the time to fewer cities, as DL was much smaller. (They did not fly to BUF or to Rochester MN), but they did go to SJC, MCO, ORD,PIT, SLC, JFK, LGA, DFW, CVG, and many more airports that I now remember fondly. I once baby sat my brothers kids in SJC over the period of 6 weekends, by going back and forth each Fri-Sun with this, and only once almost missed the last flight back.

I should have joined the FF program then, but I guess the pass did not earn miles (I never did find out)