Hi, Hoyt! This was kind of a tough question, because I cannot remember ever having to convince my family to go to Walt Disney World!
Have you thought about sending them the Walt Disney World vacation planning video? The DVD will give them a complete tour of the four theme parks, take them through the resort options, and show them all the fun stuff there is to do at Walt Disney World even if they don't want to go to a theme park. You can find an order form for the Planning DVD by visiting here
Walt Disney World Vacation Planning DVD.
Another thing I like to do with my girls when we've gone too long between vacations is to plan a "Disney night" at our house. We'll pull out some Disney themed games, check online for some of the recipes from the Walt Disney World resort, and we'll cook a nice Disney dinner, play the games, and maybe watch a Disney movie. They get so excited being reminded of the great food and fun characters!
I hope you can convince them to make the trip with you! I can't think of anything more magical than making family memories at Walt Disney World!