When something is really fun, you can say that it's "a blast".
A: Have you been to the State Fair?
B: No, I haven't.
A: You should go. It's a blast!
Your party was a blast. Thanks so much for inviting me!
There's also the phrase "have a blast":
A: How was Argentina?
B: I had a blast!
"A blast" is OK to use whenever you are enthusiastic and excited. It's OK in both formal and casual situations.