Hi there Deanna,
Welcome to planDisney and thanks so much for these fantastic questions! It looks like your trip to the Disneyland Resort is coming up at the end of this month - how exciting!
Your
theme park ticket and
park reservation are all you need to enter the parks throughout the day - that includes the ability to leave and re-enter as needed. That is a super common occurrence for me and my family during our trips to the Happiest Place on Earth. We'll often leave a park after lunch for a little bit of downtime back at the hotel before we head back in for the evening. Keep in mind that you end up leaving the Disneyland Resort area through the security screening gates then you will need to go back through that checkpoint when you return.
As for backpacks, there are some general guidelines that you can find on the
Park Rules & Regulations page. In general, this will be a great page to review for all the items that are allowed into the parks. Specifically the page says "Suitcases, bags, coolers or backpacks, with or without wheels, larger than 24" long x 15" wide x 18" high (61 cm x 38 cm x 46 cm) are not allowed in any theme park."
Lastly, you can bring some snacks and water bottles but keep in mind that ultimately it is up to the discretion of the cast members on what is allowed into the parks. On my visits, we have brought snacks for our kids like pretzels, chips, trail mix, and water bottles.
Thanks again for your question, Deanna! I hope you have a fun time on your upcoming trip to the Happiest Place on Earth.
Have a Magical Day,
Greg