Never heard the whiskey thing, but I have a friend whose horse will stop sweating in the summer so the vet told her to put a can of beer in his food every morning. He loves it, and definitely helped him start sweating again.
I always give carrots as treats. Just cut them up into small pieces and reward sparingly. Plus they're pretty cheap.
since apples have a lot of sugar, I cut up carrots into sugar cube sized pieces and give my horse quite a bit everyday. He is very food motivated so I reward him often with carrot bits, especially when learning something new. And the carrots are fairly cheap and not messy which makes them even better.
Actually carrots are also high in sugar............I know this because I have a PSSM guy and it is on the list of things high in sugar.
I think alfalfa cubes or pellets make great treats.....alfalfa is low in sugar.
I had a horse who would go for about any fruit... grapes, pears, whatever, but watermelon was his favorite, though. He'd eat the rind and all. He also loved slices of whole wheat bread every now and again... he always got the "heel" slices off of the grocery store loaves.
Probably not the healthiest or the most sugar free, but I've seen worse fed to horses.