This is something the King Bee thinks about all the time, and most likely over-thinks it every time.
Its a real balancing act, Be prepared for whatever may cross your path, but don't over-load yourself to the point of ruining a fun lazy day. You want to make sure you never have to say "gawddammit I wish I'd remember to bring..." but you also don't want that lame feeling of coming home and unpacking all the supplies you wracked your brain to gather the day before.
So whats the secret?! Consider your base needs. You need water, you protein, and you need toilet paper. That last one seems kinda silly, you'll get home and say out loud "Well I didn't need THAT much water", and you can always grab a burger on the way home if you didnt bring enough food, but no ones ever been filled with regret for remembering to bring a roll of T.P. If I need to explain this, I cant stand near you. Thanks.
So, how about your water? The short answer is about 2.5-3 liters of water. Now some websites are gonna say way more and others say way less, but the way I figure it, you're splashing in the water, shade is available when you're taking a break and you're never really terribly far from civilization or assistance if the day goes wrong so I feel this is a safe number. I typically keep a couple spare bottles on my 'yak anyway, just in case. they wont be cold but they will get you where you need to be.
Food? Listen, I love a big meal but I tend to run a little lean when I'm planning a full day of paddling up and down the river so I never weigh myself down too much. 3-4 Clif Bars and a half dozen of those little oranges and I'm good. I've had pals that were super gung-ho about hamburger cookouts on the bank of the river, loading up their campfire grills, a king size cooler with two dozen burgers, and enough "fixins" to put a fancy build-your-burger joint to shame. Don't be that guy, if you gotta be the king of camp cuisine, a pack of jumbo dogs, a bag of buns, and all the condiment packets you can lift from the gas station on the way out will suffice. Cut a stick off a tree, warm up your dog over the fire, call it good.
Sunblock? This takes a few different forms, obviously you need some good 75-100 proof water resistant stuff, you're going to sweat most of it off so re-apply regularly, and just a tip from personal experience, take off your shoes and apply that stuff well below the line of your shoe, Its a terrible place to burn and its often overlooked. Sunglasses are mandatory, no one wants to squint that long, and I'm a huge proponent of oversized straw hats. Keep that sun off your head face and shoulders. A ball cap just isn't going to cut it. Heck, get one of those Asian coolie hats, if anybody gives you shit about cultural appropriation you tell them "the King Bee says Fuck off!"
Survival tools. It's easy to fall down the rabbit hole with this one. In a worst case scenario, you need to hike back to the road, not rebuild civilization amidst Mad Max level societal collapse. So whats important? Well we jump right back to water for this one. But if you feel good about that, next Id say basic first aid in a waterproof pouch. Bandages, disinfectants, etc. Saline solution for the guy that gets sunblock in his eye, lip balm, spare sunblock allergy pills, a pocket knife, signal mirror, some rope... See how easy it is?! This list can just blow up if you let it. That being said, the goal is "get back to the road" and its not tough to do if you're in a well traveled area like the lower Salt River so just be smart.
Frivolous stuff? I like a nice folding chair, maybe a good waterproof bluetooth speaker or headphones. Its nice to own a good medium sized cooler that keeps a six of cheap beer real cold, doubles as a foot rest if you want to kick back and watch the river flow from the share of an old mesquite tree.
You came out here to hide from your daily hassle, so just keep that in mind and remember to leave the hassle behind. Keep it simple, enjoy the sun on your face and the sound of the water and it splashes along. Take it in, it wont be around forever!