What should I pack in a day-bag for Disneyland?

The smallest one possible. Some people bring in full on back packs with hip straps. I’ve seen purses, canvas Ikea bags, reusable shopping bags, tote bags, draw string back packs, fanny packs etc. I prefer a very small woven cloth back pack (think the size of a fanny pack). It has comfortable wide shoulder straps, is small enough to carry by one hand if needed, doesn’t get in the way or bump people in crowds, easily fits in the on-ride back pouches, and stretches to accommodate weird shapped items. It’s also gender neutral so anyone in my group can carry it.

As little as possible. Yup. That’s it. The bag will get in the way, get heavy, and be an easy item to forget somewhere. If you can get away without it, do it. I’ve only entered any Disney park once without a bag, and that was because I knew I was only going to be there for a few hours. For day long trips, I always end up with a bag. Here’s what’s in it.
-Sunscreen
-Note cards with daily plans on oneside, food/snack “bucket” lists on the other.
-Small collapsable re-fillable water bottle
-Wallet
-Hotel room key
-Cell Phone
-Pocket camera(I still prefer this to a cell phone)
-Small snack- Yes there is food almost everywhere at Disneyland, but sometimes I get stuck somewhere (like a long attraction line) that doesn’t have food, or there isn’t anything I want nearby, and I know I need to eat a quick snack before turning into a Snickers commercial. I almost never use this item, good planning of meal/snack breaks help, but it’s there just in case. Nothing seems to turn a good day into a bad day faster than a hunger meltdown.
-Hand sanitizer attached to the outside of the bag. Use it often.

-Wet wipes. Small, light and come in handy. Two individually packaged wipes can be helpful for sticky hands after a chocolate covered Mickey cookie. No need to hunt down a bathroom just to get the smuggled chocolate off your fingers.
-I use a lanyard and attach it to the strap of the bag for easy access. Inside the plastic holder I put my Passport(ticket), Fastpasses, and any Photopass+ cards I receive from Photopass photographers.

Yes that is really it. If the weather looks really iffy, I’ll put a $1.00 poncho in the bottle of the bag. I’ve yet to use one. It almost never rains at Disneyland.
Some items get tossed in due to situations. If I wake up with a headache, I’ll toss a small container of headache medicine in the bag. Breath mints if I think I’ll need them. etc. I don’t go overboard with the first-aid kits. The park sells basic stuff in the large stores, and you can always head to first aid if you need to.
I do sometimes carry a light windbreaker. It folds up small enough to tuck into the strap of the bag. Most of the time I bring it just in case, and never use it.

You won’t. I don’t bother to carry a huge nice camera into the park. Why? Well my main reason, I want to experience my trip, and spend less time taking pictures. A cell phone and a point and shoot camera do it all for me. I know some people really enjoy taking pictures (I’m looking at you Tom Bricker) when they are at the theme parks. I want pictures to capture memories of the trip, but over the years, I’ve found I spend way more time remembering the trip, then I ever do looking at pictures of the trip. I want mental pictures, so I take less digital ones. The normal reasons not to carry a large camera, theft, damage, losing it also apply. If you have a nice camera and you plan to bring it into the parks, you will need a slightly larger bag probably with some padded protection. Try a day with your trusty smart phone, and see if you really do miss that camera. If you don’t then just leave it at home next time.

Do you really want to carry around several extra pounds of stuff, just in case something rare happens? To me, its worth it to have a lighter bag, and know I may have to pay theme park prices for tylenol in the event a sudden headache occurs.

All the normal stuff you’d pack for an entire day out. You’re probably going to be bringing a stroller (props to you if you just use a baby carrier) so you can load it up like a sherpa. Don’t forgot a blanket to cover the stroller with for sun protections, nap times etc.

Same stuff you pack for a day trip with a toddler. Snacks, toys, a lovey(best stuffed animal friend) a blanket for a nap time, a few changes of clothes(First Mickey bars are messy) sunscreen, bottle, etc. also possibly moleskin for blisters. If you’ve worn in your shoes and prepped your feet well with good socks etc you probably won’t get blisters, but your 2 yearold might not be able to tell you that their feet hurt until it’s way to late. Second pair of shoes for blister relief, mud puddle finds, sticky ice cream dumps, etc. A second pair of socks for extra sweaty feet.

All the items on the adult list and WAY more snacks. Unless you want to shell out for park snacks every time they are hungry, load up. You can just use a bigger bag, or make everyone carry their own small bag. Also each person needs a water bottle of their own. They may want their own bag complete with cell phone. Kids seem to want to spend all the time in line on their phone, but it can mean missing out on fun family time. Plan ahead and make up some fun games(or search off the internet) for fun scavenger hunts to do while in line. Play the Heads Up app, plan what the next ride will be, talk about the queue. You may also want to toss in a deck of cards, or other small toys/games. A mini key chain sized Etch-e-sketch doesn’t take up much room, but can be easy entertainment for the entire line for Pirates Of The Caribbean.

All the normal adult items apply here. Don’t forget the need for extra hygiene items. Lotion for dry hands, eye drops etc. Any medications should be packed. Extra layers of cloths if you tend to get cold easy.