If you want to see everything, get a touring plan (around £15 from touring plans.com)
You might be able to park in Disney hotels and use the free shuttles to get to and from the park. This is worth it for MK as you will be taken straight to the park and cuts out the monorail, however might be easier to just pay the $20 for the other parks...
If you have small kids, make use of the rider swap. Combine with fast passes to double the fast pass rides you can use.
As pp said, free ice water anywhere the serve fountain drinks.
Buy a bucket of popcorn for $10 ish and have refills for $3ish (sorry can't remember exact amounts) they do different flavours in each park so can try them all :)
There's a battery power bank you can buy, that when it is out of power, you go to a charging station and swap it for a fully charged one. Then keep doing that the whole holiday. Haven't used this myself but heard it works fine with no fuss.
If celebrating anything, go to a shop for a free badge. My daughter got a ton of free cakes and deserts and general well wishes when she wore her Disney birthday badge :)
You can go to wholesalers near by and buy discounted gift cards (their discounted by only around 5% so I would only do this if you are going there anyway.) they can be used to buy food/drinks/toys etc
I probably have more, just can't think of them right now! I live 3 hours from Orlando and so have an annual pass... We have just done our eighth trip since August 😁