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.