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Cater to everyone's interests
This may fall into the category of "obvious," but believe it or not there are those who think: "Well, if I like deep sea fishing off the coast of Maine in November, everybody must!" Beaches, theme parks, cities and national parks all have a wide variety of activities and interests the whole family can enjoy.
But here's how to have a foolproof vacation: Once you've decided on a destination, have every member of the family write down what they want to do there, whether it's learning to paddleboard, or going on a long hike, or seeing the world's largest ball of twine.
Armed with that information, you can then create an itinerary by doing exactly what members want, merging certain activities that are similar or trying some things that are completely new to everyone. This wish list will let everyone feel their desires have been heard, and they'll be more likely to enjoy other's recommendations.
Take weather into account
As an experienced traveler, I prefer off-season travel for its lack of crowds and better pricing. I think it's a fine trade-off, even if it means the likelihood of colder, hotter, or more rainy days.
But for children and even teenagers, who aren't paying the bills, this might cause more moodiness and meltdowns. So for multigenerational trips I recommend sticking to the high season — or close to it.