I believe the word "really" is quite an adequate expression for this situation, but would there be other alternative words that can replace it?

Rebecca

Alternative words to replace "really" as an exclamation would be "seriously" or "honestly", depending on the context. You could also use "for real?" which is casual and informal. Ex: I can borrow your favourite shoes? For real?! Ex: Seriously? That was an immature thing to do. Ex: I think you're a great friend. Honestly.