形容詞

I am currently fascinated by the Japanese 形容詞 楽しい. I have always understood the term to refer to a thing or event as being enjoyable, as in パアクは楽しかった. But I am now hearing it used with a personal subject as well, which seems to make it modify both the object, as stative, and the subject, as responsive at the same time. An example would be 私も（パアクが）とても楽しかった.

Having been gone from Japan for forty years now, I no longer "think" in Japanese, but this usage seems very strange to me. I know that 形容詞 are much closer to verbs in Japanese than adjectives are in English, and I am told that the expression sounds perfectly natural to the Japanese ear, but I still can't help but wonder if this is not an example of language drift resulting from common Japanese practice of leaving out both the subject and the object in spoken Japanese. Anybody care to give a newbie some feedback?

パーク/公園 is the OBJECT of 楽しかった in the first example sentence. The object is topicalized and the subject "I" is omitted. Thus, 公園は楽しかった actually means "I enjoyed a/the park". Similarly, the second example sentence 私も（公園が）とても楽しかった is "I, too, enjoyed it(= the park) very much".

が indicates the object of adjectives that express emotions/sense such like 怖い, 好きだ, 嫌いだ, 苦手だ, 痛い, state verbs such like わかる, 要る, できる, or potential forms/-tai forms of verbs.
e.g.
幽霊が怖い
I fear ghosts.

料理が苦手だ
I'm not good at cooking.

英語がわかる
I can understand English.

旅行するのにお金が要る
I need money to go to a trip.

英語が話せる
I can speak English.

寿司が食べたい
I want to eat sushi.

As close as 形容詞 are to verbs, I assume that we have to say, at some point, that they are not verbs. So please explain to me the difference between 私はパアクを楽しむ and 私はパアクが楽しい. Thank you for your feed back, by the way. I really do want to get a handle on this. I don't believe, for example, that anyone would ever try to say (私は）本がおもしろい.

楽しい is a state(since it's an adjective) and 楽しむ is a volitional action. That's the difference. Notice that 楽しかった expresses an actual experience "was in a state of enjoying/pleasant", so it's close to 楽しんだ in meaning.

"本がおもしろい" - Google 検索

(BTW, パアク is not common, or rather, looks odd to me.)

So the language drift is on me. What I always understood to mean (without the subject being expressed) as "something that is enjoyable," much as おもしろい means "something that is interesting," actually is a state and not an attribution. Which leads me to my next question: Is 楽しい in any way being used as a replacement for what I figure is now considered 文語体 the expression 幸せ? I see 私は楽しい translated as "I am happy," but, without any context, I don't know how literally to take that.

幸せ is not a 文語体 at all. It's quote commonly used in daily life.
I'm in a state of "being happy" doesn't make sense?