I have a love/hate relationship with "hack days". Back when I was an intern I had a lot of fun during hack days. Getting to see a new part of the codebase and getting to interact with new teams is always fun.

When I was working full time, my idea of "hack days" changed quite a bit. I took it as time to work on the weird idea in the back of my mind. My demo's were not impressive by any means but I learnt quite a bit out of it. I also think its part of the team dynamics, some teams embrace hack days while others do not. Think if the tech lead of a team actively encourages hack days the vibes would be completely different say compared to just doing your day job and planned activities.