On our last day trip, when I was going with just myself and my two DDs, I brought a fully loaded backpack (the type you would use for a day hike or school). And I mean fully loaded! It had those little plastic apples of Marinelli's apple cider, individual Silk Chocoloate Soy milk cartons, half-sandwiches, Lara bars, bags of pretzels, and bags of Snackimal cookies. I was determined to avoid spending tons of $ on snacks/food for a one day trip. And I know my girls, I knew they would ask for a snack each time they saw a churro cart.
That part of the backpack wasn't so bad, because as the day wore on, the load got lighter (though we didn't go through nearly as much of the stuff as I thought we would). I also had in the backpack jackets for when it got colder, my girls' pin lanyards (because they only wear them when they see a CM with pins and then they hand them back to me 10 minutes later), extra socks in case we got wet, our autograph book, and coloring stuff for downtime. And, obviously, it had my stuff like phone/ID/money well tucked away inside an interior pocket.
I was completeley prepared to carry it around all day and I did for the first hour or so that we were there. Despite a fairly loaded pack, it was really easy to load on a ride, even on the Fantasyland rides with all three of us riding together. And it was pretty comfortable. The reason I didn't carry it around the rest of the day is because DH showed up unexpectedly to surprise us!

He took it over, because that's just what he does. But, he didn't seem to mind it, either. Much easier than lugging around a stroller and trying to park it all day (not that is something the OP would do, but just FYI for others with younger children...my youngest was 3 in February and did just fine walking all day).