Was mache ich in meiner Freizetit mache

'In' without direction --> dative (--> 'meineR')
Then, as the title was originally worded, it was a question, I feel like it makes more sense to put the 'mache' to the end, making it a subordinate clause

Aber wenn habe ich, ich nutze esich welche habe, nutze ich sie sehr gerne bis zum Maximum aus.

The subordinate clause at the start of the sentence counts as being in position 1 of the main clause, therefore the verb has to be in the first actual position in the main clause.
What was 'es' in your text refers to 'Freizeit' which is female --> 'sie'.

Ich backe viele Kekse und Kuchen auf (mein Lieblingsgeschmack ist Schokolade).

Your original sentence was also correct, but it would mean that you just warm up pre-baked cakes/cookies in the oven, instead of actually baking them.

Ich gehe auch gern spazieren, ins Kino gehen und meine Freunde treffen.

Ich mache ein bisschen Sport, besonders Halnteln anheben.

I must admit, I am not all too familiar with workout related vocabulary in my native language, but I do suspect that 'Hanteln anheben' is not the correct term.