On the few occasions DH and I have gone alone we've found that we can carry what we need and that we don't need a backpack. We usually each have a waterbottle, our phones, a camera, and the tickets in my lanyard pouch. We used to take a backpack, and have found that it can be hot and sometimes a pain.
Most of our trips we have our kids. Sometimes we bring a backpack, but a lot of times we use the stroller and our pockets. Diapers, a blanket, food and water all go in the bottom of the stroller. Sometimes we throw in flip flops for water rides. Then the phones, camera, etc. listed above can be carried by DH and I. We are not worried about diapers or food being stolen from our stroller and just make sure we have anything of value (ie. phones, camera) on our bodies. The only downside to this is we sometimes find we want something like water or snack while in line and we left it in the stroller when we parked it, but usually we remember to grab the things we need. When our youngest grows out of a stroller in a few years, we'll have to go back to a backpack. But at least we won't need diapers.