Is there any sugar-substitute in these?

Cathy, yes, it's Sucralose. Out of the 13 listed ingredients, Sucralose is the 11th, so there isn't a very high concentration of it. There's no fat of any kind, 55 mg. of sodium, 45 mg. potassium & 100% vitamin C per stick. Honestly, they taste very good! You mix 1 stick into either a 16 oz. or 20 oz. bottle of water. I prefer the 20 oz., 16 oz. seems too fruity-sweet to me. I'm ordering another flavor to try in a while. Hope this lengthy answer helps you!

What are the exact ingredients?

Ingredients: Citric acid, maltodextrin, potassium citrate, salt, ascorbic acid (vitamin C), sodium citrate, potassium phosphate, sodium phosphate, sucralose, yellow 6, natural flavor and calcium phosphate.

Citrus and lemon-lime, both are cool and refreshing. Raspberry and mixed berry are totally delicous if you prefer berry flavors. Orange is also very good. I found grape to be too light/no flavor unless you use little water or mix two together. Strawberry-lemonade is also very delicious and taste just like its name.

is there any kind of stimulant in the ingredients?

I did not see any on the label of ingredients. This is not a product I have found readily in stores I actually got my first taste from my employer at the time for hydration (Sugared variety syrup)