more casually, "진짜??" or "놀랍다/놀랍네"

Depending on the contexts, but here are some examples: 듣고보니 놀랍네! (Once I hear about it, it's surprising!) 뜻밖이네! (What an unexpected thing I hear!) 와, 진짜? / 와, 정말? (Really?) 진짜 생각도 못했는데. (I haven't even thought about it...)