Me personally: I hiked the Pacific Crest Trail from the Mexico-California border to the Washington-Canada border in 2022. While I missed work on trail, it definitely made me realize that once I'm financially independent, I'm mostly going to be living a life outdoors, with occasional indoor time to play music.
Your situation is a bit different from my goals, but honestly if I had that opportunity, I'd be doing my best to explore the woods/mountains nearby in whatever way I can.
I also kind of agree with the "Learn German" comment - learning a non engineering skill that's also either practical or brings you joy is a great way to eat up time. I'm speak a few languages, but as I only regularly use English in my day-to-day life, just maintaining the other ones at a high skill level is a time consumer, though one I enjoy doing.