Magic Kingdom: not really a secret but apparently most people start their day going right from the hub into Tomorrowland and not at Adventureland (that why the bridge to Tomorrowland is wider and bigger than the one to Adventureland) so if you want short waits try starting at Adventureland and make your way over to Tomorrowland last!
I can’t attest to this. Usually can do everything in Adventureland and Frontierland including Haunted Mansion before lunch using no fast passes. Actually we go through Liberty Square to HM then down the lake to Splash and Thunder. All usually have a wait under 20 minutes. Then I to Adventureland and Pirates is about the same wait. This allows you to use fast passes for 3-5 for the big rides and cooling off in a show or two.