Our biggest mistake has been not planning our days. Our first trip to DW was heavily researched, planned and super fun. The second trip, we figured that we knew what we were doing and planned much less. Once there, we found ourselves spending more time trying to figure out where we wanted to be, what we wanted to do, and where we should eat. Every moment was another decision to make, so we spent much less time really enjoying anything.
So now, we heavily plan (yes, detailed itinerary) and then adjust or blow up the plan as the day evolves. And now that we have kids, having a plan is even more critical. For our family, going into a day with a plan of action makes it a much more relaxing adventure.