Mild:
* Awesome!
* Cool!
* Great!
* Sweet!
* That's so exciting!
* I'm pumped!
* I'm stoked!
* Can't wait!
* I'm in!
Moderate:
* Woohoo!
* Yeah!
* Let's go!
* Heck yeah!
* I'm all for it!
* This is going to be epic!
Extreme:
* Holy moly!
* OMG!
* I'm freaking out!
* This is the best thing ever!
* I'm going to explode!
* I'm so excited, I could cry!
The best expression to use will depend on the situation and the person you are speaking to.