Keeping a sick cat hydrated?

Gus, our little terror with food allergies, managed to find a granola bar and ate the entire thing. This was sometime between 9 and 11am yesterday. He threw up at about 1pm [yesterday], and it looked like the entire granola bar came up. He continued to throw up liquid every 3 hours approximately the rest of the day, through the night. We held his food last night, but fed him about 1/2 a can with additional water mixed in....and he threw that up a couple of hours later. He seems alright at this point. I checked the skin on the back of his neck, gum color...all seem normal, but I would hate for him to end up stuck in a clinic getting fluids.

Are there any ways I can help keep him hydrated? I thought about making a broth, but I'm not sure with what as I don't to upset his stomach further.

What I do is open a can of WATER packed tuna and mix about half water and half tuna water, this is seldom turned down, don't put any of the tuna with it as this may upset the tummy more. As I use the tuna water I fill the can with more water and let it sit until next time then pour it off and mix with more water. That way the can may last a couple of days, I then throw the tuna away as it pretty much a mushy mess May seem a waste but it keeps my girls out of the vet's office most of the time and a vet visit costs a lot more than a can of tuna

I hydrate with a eye dropper or similar, just plain water.
Squirt or drop water to opposite side in the mouth.
Can you hold your kitty to hydrate him by hand yourself?
Always offer treat after this.

When my PurrBoyz were teens and got very sick from a reaction to a common anti-worm medicine, they both began to get really dehydrated (they were half Siamese and sensitive to the med, my female Heinz57 Tabby was unaffected).  They were still trying to eat, but refused to drink any water!  Both got really ill, and one started going blind and had a convulsion.  I took away all of their solid food, kept fresh water available, and fed them only a liquid diet of tuna water or canned chicken water for a week, even getting up every four hours to give them some more.  I did the same thing you did, except I dumped my tuna or chicken into a tight container, added water, and kept it in the refrigerator.  The tuna or chicken was basically just a flavoring to get them to drink.  Since they were hungry and had no food, they readily drank the tuna or chicken water for their meals.  I am so thankful this solution was given to me, because they both recovered completely.  I am using tuna water right now for a sick cat  who keeps coughing and throwing up solid food. I am actually giving him mashed up watery tuna since he has to have an antibiotic mixed in. So far this is helping him and he hasn't thrown it up, thank the Lord!