Figure out what year your models were made and then go hit a library with magazines archives (Macworld for one) and check out what was advertised in the issues of that year. Whatever they used it for then you can still use it for now with the added benefit of having cheap RAM and HD upgrades that were very pricey when the machines were new.

There should be tons of word processing, database, office, publishing, video editing, games, etc that would run well on those machines.