Reddit mentions of Osprey Daylite Backpack, Alpine Blue, O/S

I have used a few different backpacks when going across the parks for many years. Here is some insight and lessons I have picked up over the years.

Tip 1The more you pack, the more time you spend waiting to get through bag check each time you go to enter a park. It helps if there are ample pockets or an easy way for security to shine a light to the bottom of the bag to make sure things look ok. Always pre-stage your bag on the security counter by unzipping all compartments and pull out any large bulky items at the top that impede viewing to the bottom. It is a rushed process but if you need to take you time just apologize to the person in line behind you. Once you can get your bag relatively secure without losing anything, move out of the way, 15-20 steps past security and reassemble anything out or order. Just courtesy.

Using a 'normal back pack' like the one pictured is great for lighter loads. The reason being the straps hang off the shoulder. It is also easy to loop straps through the legs for thrill rides, and is small enough for being stuffed in mesh pockets on Dinosaur and Soarin' over California. However, the weight is not distributed across the shoulders. So you either have to take it off periodically or switch carrying duties as it will get heavy.

More 'Advanced day packs' like the one pictures have cross straps in the front that adjust and buckle. These are more expensive but provide additional features that greatly enhance your day in the parks. They usually are a little bit larger, so you can take more stuff, and have compartments so you can put diapers in one side, snacks in another, and electronics in yet another one. The cross straps position the bag a few different ways. The waist strap is fantastic since it allows the bag to not hang its weight on your shoulders but across your hips, making it easier to do a full day. This can also allow you to get the back of the bag off your back, allowing for some air to pass through keeping you cool. The shoulder cross strap aides in keeping the shoulders from being over loaded. A slight con with using these bags are that the cross straps provide additional straps in which it can be difficult to quickly remove the bag or to stow it properly without straps going all over the place.

You are familiar with the 'hiking packpack'. They feature some sort of metal frame. They come in a multitude of sizes and features. These are the "I'm wearing it all day and carrying 50 pounds of stuff" bags. They hold their shape well and provide the most support with regards to weight distributing. However, they don't fit particularly well on most attractions due to size and the non-collapsing nature. Sure you can get on Winnie the Pooh, Peterpan, and stage shows with no issues, except that you won't have any leg room for 3 minutes. I have seen people bring these to the parks and make use of them but I venture to say they are uncommon.

Tip 2 Disney does not have bag holding areas with their attractions. They do not want to assume responsibility for anything. People for many years have left bags with strollers in the parks, so you can do that at your own risk. Rides either are calm enough to allow you to hold bags in your lap or on the floor. Or they have some sort of mesh pouch, or allow you to loop through your legs. Chances are for the intense rides, you both won't be on it at the same time because of child swap, so bag size won't matter.

tl;dr
My favorite for heavy days is the day pack.