We bring a lot, but we're also packing differently than we would if it were just DH and I.
In a backpack goes:
Diapers/Wipes, water bottles (we bring 6 full sized Contigo/Camelback water bottles but only 2 go into the backpack), extra clothing for the 3 littlest, snacks, camera, tickets, room keys, a smaller bag with necessities (lip balm, diaper cream, sunscreen, bandaids, first aid kit, etc...), Ponchos, umbrella, . Everything has it's own pocket or is put into a smaller bag.
Then we have an extra bag that we put our lunches into (we love bento boxes for lunches). That hangs from the stroller. Then under the stroller goes any water bottle that wouldn't fit into the backpack. As we eat lunch we stack the bento boxes, which makes room in the large bag for anything we buy.
I also always have a wrap (or two) with me that we can carry the babies in, then use as a blanket or picnic blanket as needed. I've even used them as towels before. We have a Valco Twin Runabout triple stroller, so plenty of space to hang things from the back or put underneath. My favorite bags for Disney (just in case any other moms are looking for an awesome disney diaper bag backpack) are from Ju-Ju-Be. They have a backpack named BeRightBack (BRB) that holds everything, still has room for more and never gets heavy or uncomfortable. Highly recommended if you are looking for something to take into the parks with kids. They aren't cheap, but very well made and last and last. Then when we stop for a ride or something I grab the backpack from the stroller and everything else stays with it.