I don't want to be productive just for the sake of being productive. What I meant is that I have some "hobbies" that I know I enjoy doing in right circumstances, having enough mental energy being one of them.
Perhaps stop looking for the "right circumstances" to pursue your hobbies and just start engaging in them. Even if its only for 15 minutes a day. My hobbies restore my mental energy. If I get tired of coding, I go out in the woodshop and make something. Then I come back to coding and I'm more productive. Make sure your hobby is something orthogonal to what you do for work.