A lot of people "pre-shop" their souviniers. The Dollar Store often has little Disney toys or coloring books - plus glow bracelets.
WalMart has been known to carry Mickey T shirts. Pack them and pull them out at appropriate times. If you need a "park memory" outside of the photos you take, choose one thing and choose wisely (I like Christmas ornaments, you can find them pretty inexpensively in the parks). Remember at home it becomes "plastic junk you trip over."
If you purchase snacks, split them. A Dole Whip is a wonderful thing, be we find one can be split four ways and everyone gets enough of a taste.
Some people leave their coolers in the car.
Remember if you park, your pass is good for the day if you leave and come back.
There are few expenses you NEED to budget for - parking is one if you are staying offsite. You need to eat, but you can pack in inexpensive food (Disney doesn't allow rolling coolers because they are easy for people to trip over - and lockers are expensive, so plan on carrying what you haul in). You won't be the first Disney guests to eat peanut butter sandwiches.
A lot of people bring those little packets of crystal light and ask for water - which any counter service place will give you. Many people don't like the tap water at Disney (its a little sulfurous). There are also ample water fountains if you bring a water bottle.
If you need a stroller bring it - if its marginal, bring it anyway. Make sure to pack everything you need or might need - sunscreen, extra socks, bandaids, feminine products, advil, a hat, sunglasses (think sun, walking, blisters) - and carry a good selection of that stuff into the parks. Nothing blows the budget like arriving in the parks for the day and spending $13 on sunscreen because you forgot to grab it or realizing you didn't bring the charger for the camera or having to buy a $11 rain poncho.