Those really cheap cruises like $299 for 5 days or a "trip to nowhere" for even less tend to pop up closer to the date they usually happen--maybe a month out or less. VacationsToGo has a "90 day ticker" that shows a lot of severe reductions in pice once it gets within 90 days, and sometimes you can find the same cruise for even less elsewhere. VacationsToGo is a nice format to look at, though, so it's a good place to start.

Can you promise your parents an unspecified cruise and then fill in the details later?