Don't get hopping or waterpark admissions on your tickets.
Don't go crazy with souvenir buying (we never spend more than $25 each for DH and me and DD saves her own $$ to bring).
Pack breakfast food and snacks to bridge the gap between Free QS dining allotments and what you want to eat. We usually bring our own b'fasts and extra snacks so we don't need to spend any extra $$ in the parks for anything not covered by Free Dining.
Take advantage of free cups of ice water at QS restaurants rather than buying bottled.
Since you are driving, stuff your bags with all conceivable stuff you might need that is expensive at WDW- cold meds, sunscreen, beach towels, extra batteries, etc, etc. Won't cost you anything extra to bring it along. If you don't need it while you are there, cart it back home after the vacation.
Buy "Mickey Gifts" for kids before your trip at
Disney Store or Dollar Store- things like glow bracelets, stuffed animals, T shirts, spinner toys, coloring books, etc, etc- and leave them on their pillows every day. Cuts down on the "I want..."s at the parks.