Translation:Guitar practice was really fun.
楽しかった is the past form of 楽しい. adjectives do have their own conjugation and い-adjectives lose their trailing い and get かった to form past tense. the copula です only brings politeness to the sentence, it doesn't bear any past/non-past function here. it could have been 楽しかったです instead of the plain 楽しかった.
being myself a fellow Jp learner, the only time I've seen です in its past form でした so far is when it is paired with ありません. because ありません not being able to take both negative and past forms, it becomes ~くありませんでした.
[edit: です becomes でした in past tense when used with な adjectives]