I'll usually make sure I'm well hydrated before getting to the course and then I'll have a Gator Aid before I start my round.  After that, I just continue to drink a lot of water.  I will drink a second G.A. at the beginning of the back nine if it's really hot and I'm sweating a lot.   I also cary an extra towel or two which I'll keep wet and use to wipe my hands with (in the StL hot humid summers my hands tend to get "sticky" from sweating during a round).  As for food, depending on the time of day I'm playing, I'll either have a good breakfast or a light lunch including a banana, which will usually hold me through my round.

I bring my Titleist microfiber player's towel. I keep the towel damp and wrap it around my neck, against the carotid arteries (not too tight for obvious reasons). I usually hold part of it with my non-glove hand to prevent it from sweating. I find it easier to dry a hand that is wet with water than one that is wet with sweat. Food wise, I go with bananas for potassium and some kind of electrolyte replenishing beverage.

For sure a couple extra towels. I keep one wet to wipe off grips. A couple of gloves to alternate as they get wet. Cool moisture wicking clothes and a frog togg towel to help cool my skin. Try not to eat heavy, usually a couple of protein type snacks like almonds.

For me I really like an energy and electrolyte replacement mix called ZipFizz. Just mix a tube with a bottle of water; very refreshing.  The Frogger towels also work wonders and helps keep you cool.

I know you said beside water but water is the key,I have been thinking of trying some coconut water,I heard nothing but good reviews about it.I usually bring along bananas and  some raisins and a salted trail mix. along with a stash of protein bars.

The heat is the killer here in the Arizona valley.  Water is the only tool I use.  Salt will kill you.  Extra gloves every 9 holes.  Cool towels help and having to get up at 4 am before it hits 90 before sunrise.  Oh, well...this too will pass... Good luck!

I have a bag that has a built in cooler I would make sure to put a ice pack in there to keep food like granola bars and water cool.

 one wet towel ,one dry and cooler with 3 waters and 3 beers ,I  alternate between both to keep fluids correct

Chris -

Water is the key.  I add apple juice or a supplement called NUUN [‘all day’; ‘active’ if it is very hot] to my water.  Drink enough water so that you are hydrated before arriving at the course:  a cup (8 oz.) when you wake up and another cup by the time you leave the house.  

Granola bars are in the bag, too.  Also, stretch often.  

A well-known golf exercise (yoga) expert came to town and suggested the following:

1. drink so that you consume 16 ounces of water in six holes, and;
2. lightly stretch (shoulders, glutes, hamstrings) every three holes.

https://nuun.com/products/

Extra towel and always drink water before.  Waiting until heat is present to drink water, and you're behind.  I also do no calorie powerade also.