I have found that less is more. If you find that you absolutely NEED something you did not bring, you can usually buy it in the parks. I don't mind risking having to pay Disney prices for something that I'll likely never need if it means I'll have less to carry around all vacation.
Since we have a little one, we bring diapers and baby wipes. I try to only bring as many diapers as I'll really need. Sunscreen is a must too. I just got some little sample sized ones from Walgreens to take along this year. And our cell phones.
Also, sippie cups for the girls (which we re-use/refill). My SLR camera & additional lens plus the point & shoot (for the 5 yr old to use). Sometimes the video camera too (it's small).
And yes, dad's who wear cargo shorts can carry quite a bit! And I am looking for decent women's cargos for myself...so I don't loose my phone & money (last year I realized my shorts pockets are very shallow & when I sit down my phone & cash always wanted to pop out. I lost $20 in a bathroom and almost lost my phone in the water on It's a Small World!

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We generally have a small, sling style bag & my camera case (it is like an oversized fanny pack, you can wear around your waist). Since we have a 2 year old, we still need a stroller and these items are shoved in the stroller basket until we need them or go on a ride. I like to wear my resort card, AP, credit card, etc. around my neck in an ID badge type holder).
And our iPhones have been AWESOME for so many things, not the least of which is entertaining the kids during long waits. (Watch a cartoon, listen to music, play connect four or tic tac toe...I love it!)