The two best ones that I remember:

At one point the Pan Am shuttle (BOS-LGA-DCA) had a promo in 1988 that was something like this.........

2500 miles per segment
Get a 5000 mile bonus for a same day round trip
5000 miles per segment if one flew more than 10 segments in the month
A 20000 mile bonus if one flew 20 segments in the period.

Now while shuttle tickets ran something like $200 per segment back then, they also had a "student book" which cost something like $379 for ten tickets, but they were standby, but that was immaterial. I know people that bought two books, for less than $800 and spent a couple of weekends just flying back and forth and up and earned something like 120,000 miles at the time one could get two tickets in F to GIG or EZE for that mileage, and the face value of such tickets was perhaps $16,000.

Another one that some foreign friends of mine did: Several of the major airlines had standby student airpasses for foreign students, they cost something like $599 for a month and allowed for unlimited standby travel anywhere in the United States. they were NOT supposed to earn miles..............but they did..........and earned whatever bonuses were around as well. That sort of thing died out by 1991 though.....