"Must-haves" are pretty subjective, as you can see from previous responses. I tend to try and bring only the necessities, and avoid buying anything "especially for WDW." My most-recommended bring-alongs would be:
- Ziploc bags in various sizes (great for keeping your electronics dry on wet rides, storing meal leftovers, corralling loose items in the hotel room, packing up damp swimsuits or wet toothbrushes...)
- Disposable, cheap rain ponchos if you plan to go on Kali River Rapids; more hardy ponchos if the forecast calls for ample rain
- A small park bag (e.g., crossbody mini purse or similar) sufficient to hold sunglasses, phone (with MyDisneyExperience app loaded), power pack for the phone (if you'll be spending dusk-to-dawn in the parks only); a couple of tissues, a mini-First-Aid kit (a couple of Band-Aids, a couple of blister covers, sample-size tube of antibiotic ointment, headache medicine), hand sanitizer or individually-packaged wet wipes, and autograph book and marker if you plan to get character autographs)
- sunscreen (stick, spray or lotion - whatever form you prefer, keeping in mind the 3-1-1 rule if you're putting it in your carryon)
- Good, sturdy tennis shoes and/or hiking sandals - something that supports your feet well and doesn't get slippery when wet
As far as straws and the like, my family just drinks straight out of the Disney-supplied cups and glasses like a bunch of barbarians: no straws needed. We're so good at it that we hardly ever dump the entire contents of our beverages down our fronts anymore.
We don't bring water bottles with us to the parks: if we're thirsty, we just ask for a free cup of water at a CS location.
