You don't need a bag for all that stuff! Carrying it in a bag will just slow you down at the security checkpoint. Here's what I do for each of those items:
Sunglasses - on a cord around your neck. During the day, you WILL need them, badly.
Sunscreen - get one of the mini-bottles that everyone suggests (buy the highest SPF you can find - I always use SPF 45 or higher). You will need to reapply during the day, especially if you get wet from water rides, misters, or rain.
Brita water bottle - get a carrier from a sporting goods store,
WalMart, or even Dollar Tree, to carry it on a shoulder strap or belt. You will need the bottle or else you will spend a fortune on bottled water. One bottle per day will not be enough - the Brita can be refilled with water and ice free from any restaurant of food counter. You can also fill it from drinking fountains and get free ice from any drink cart.
Itinerary and info #s - Get a tiny, pocket-size notebook from Dollar Tree and write all that stuff in the notebook, then put it into your pocket. You can also use the book for making trip notes, and noting where you parked in the huge WDW parking lots, as long as you bring a pencil or pen. Look for a small pen or pencil at Dollar Tree.
Camera - if you have a small camera, get a small belt-loop camera bag from WalMart or Ritz that's just big enough to carry the camera. If it's digital, carry a spare memory card, in the protective plastic sleeve, in your pocket. If film, carry 2 or 3 spare rolls, in the plastic containers, in you pocket.
Plastic bag - Only needed if you have a big camera that doesn't fit into a camera case. If you should need a bag, you can pick up a plastic shopping bag at any gift shop, or if you prefer to carry a Ziploc with you, just put it in your pocket.