As a former child who had her parents take her out of school for a trip to Disney World, AND as a former teacher, here is my opinion.
Bear in mind, I went to a private school, so with public schools it may be different. When my parents took me out of school, they were very real with the school. Are there educational opportunities at Disney? Sure. Can you explore other cultures on a cruise? Sure. But, when you put it into a letter, to me it just comes across as really stretching. And somewhat tacky.
Just tell the truth. "My family and I have been planning this vacation for a while. My husband and I have very hectic work schedules and this was the only time of year that would work for us to enjoy a family vacation. We assure you that our son/daughter will have all his work done ahead of time. If any extra work needs to be done, we will make sure it is turned into you completed and on time. If there is anything else we need to do to prepare for his/her absence, please let us know."
As a former teacher, I found letters like that 10 times more respectable (because they were honest) than the "Disney can be an educational experience" letters. That statement is not a lie, as Disney CAN be an educational experience, but most of those letters seem like they are trying too hard.
School officials are way more likely to respect you AND excuse the absences if you are honest AND willing to accept unexcused absences. "I know these may not be excused, and if that is the case, we understand. If there is anything we can do to excuse even some of the absences, please let us know." School officials are much more likely to accept that than "These absences should be excused because he/she will explore the animal kingdom here and learn about cultures there" and list 20 reasons they should be able to go even though the REAL reason they are going is the same reason everyone goes: to have a nice vacation.