I will confirm some of the strategies already mentioned and add some more specifics from my family playbook. So I think of planning for a heavy crowd day as a two-prong strategy. First, there is strategy for the overarching shape of the day, and second there are specific tactics to use while you are in the parks.
So shape of the day strategy is to arrive early (as many have mentioned) to enter at opening. The first few hours of the day will be the best and you should collect FPs and ride the notorious long line rides while you they have shorter morning lines. Spend no time on shopping or eating during these golden morning hours. Another shape of the day strategy is that I will make a sit down lunch reservation for 11:30am as we will be hungry early and want to have a reserved table to sit and rest and eat just as the wave of heavy crowds year start to flood in. After lunch we hit another ride and then back to hotel for loooooong break. We do not return to park until 5pmish. In the evening we hit rides that are people eaters or those that tend to have shorter lines or shop while others are watching parades/fireworks.
Ok on the specific rides, here are my tactics. I try to be holding a FP while we eat lunch so that we can get right on that ride after lunch. If we are in Dland, I try to have a Splash FP with a window that opens just as we end lunch (I use the park app to clock FP times) so we can eat, ride Splash, and then back to the hotel to change out of wet clothes and nap. If we are in DCA, I do same thing for GRR. In terms of attractions to duck into if you have no FP, are in the midst of crowd mayhem, and you need an escape right away -- here are my lists. For Dland: Tiki Room, Tarzan Treehouse, Pirates if line is moving, Tom Sawyer Island (if open), Mr. Lincoln, Main Street Cinema, watch a show in Golden Horseshoe or at Mickey Magical Map. For DCA: Redwood Creek Challenge Trail, Its Tough to Be a Bug, Animation Academy, King Triton Carousel, Ariel Undersea Adventure, Boardwalk games.
