Philosophy is excellent food for thought, working through a MOOC course (MIT/Yale?) and doing the prescribed readings might strike your fancy. Happy to give more specific recs if you have some existing curiosity about a topic. Side note that it’s difficult to find people to talk to online about serious philosophical topics, the options I’ve seen (discord groups, facebook groups) usually aren’t very engaging but the reading and lectures and contemplation are engaging on your own imo.

Watch ancient history documentaries like the Fall of Civilizations or History with Cy channels on youtube (Fall of Civilizations is also/originally a podcast if you prefer). Whenever I am feeling empty of interesting thoughts this is my go to. When I watch frequently I constantly find my thoughts combining and recombining history with my current experience in a way that feels awfully close to intellectual stimulation. I also find it gives some mildly comforting perspective on current events.

Built to purpose gadgets. Getting into arduino or similar as a hobbyist can be intellectually engaging. The process of identifying something in your space you could enhance then drafting and executing a plan (including some basic programming) is kind of like a puzzle. Building things you don’t need like an LED based checklist for chores that resets every day, a pedestal that spins to give a house plant even sunlight, or a solar powered bird house might be a fun challenge.

Edit to add that if you live near a university professors will usually let you unofficially audit their class if you’re interested in a topic and have time during the week.