If you are staying in a hotel offsite you can finds coupons for 20% at EoS and 1 free ticket for 1 paid adult ticket to the Balloon
Buy drinks and snacks at the Auchan supermarket at Val d'Europe and take them with you rather than buying in the parks.
Take your own tea,coffee, cup a soup etc and a travel kettle if your hotel doesn't supply one - then you could make up a flask and carry your own hot drinks into the park.
Muffins and Cookies are cheaper at Starbucks in the Village than Cable car bakeshop and much nicer - I even buy my coffee there too as I drink it black. They give you 10% with an AP.
Eat off site at night - again VdE has a good few restaurants all much cheaper than DLP itself.