We've never been to Disneyworld in Florida or Disneyland in Paris, but we've been to Disneyland in Anaheim California lots of times. This was 8-10 years ago, but we stayed at a Best Western hotel directly across the street from the park entrance, so we could walk back and forth when we wanted with no additional transportation fees. Hotel was cheaper than Disney hotel, still very nice, and free hot breakfast in family restaurant next door every morning was included in hotel room price.
We had a backpack that we brought in with rice krispie bars, fruit bars, and granola bars (basically things that can be squished or scrunched a bit in the backpack and are still none the worse to eat!). We were never questioned about it, and we noticed lots of others bringing in food as well. We also bought a couple of the individual bottles of pop when we first got there, then refilled them with water from water fountains whenever we needed to and reused them the whole time we were there. We ate at the more expensive places during lunchtime, (such as the Blue Bayou by the Pirates ride), when the prices were less expensive. And ate at less expensive places at supper. We pretty much stayed at the park from opening time to closing time most days!
Definitely fastpasses are a plus. And also a good time to go on some of the more popular rides would be during fireworks shows and parades, as lots of people go to watch them.
If you end up going to California, at least the exchange rate will work in your favour!