Hi, sounds like you‘ve done really well recently!
I‘m by no means an expert, but whenever I have down time during working hours, I try to do three things, in order of importance.
1. Since I am still quite new to it, I have been learning as much as I can about investing. This has led me to find MMM and index funds with low OCF. This will have a great impact on my future returns and is probably the most efficient.
2. I try to keep up to date with my industry and learn something new about my field, in order to get better at it. Depends on your job, of course, but it can‘t hurt to know more.
3. I research side hustles, as you mentioned. They usually aren‘t as lucrative as people‘s main job, but might be nice for variety.