。
That wasn't very fun.
Literal
Too-much was-not-fun-[reflective-な].
A reflective comment to oneself or a companion. あまり楽しくなかった is a straight あまり~ない past-tense adjective pattern ('wasn't very fun'), and the final な is the reflective particle (distinct from the negative imperative ~な) used when musing aloud or confirming something to oneself. Both nuances share the same shape; intonation and context disambiguate.