A post upthread has just reminded me of some "fun" things we did in form time at school. For a couple of years, our form room was a Physics lab and our form teacher was head of Physics.
Obviously that had down sides and upsides, the down sides being we never had a room we could "hang out" in as the labs were locked when the form teacher wasn't there, so we always had to hang out in the corridor and had nowhere to put our coats, bags, store books, etc., in the same way that other forms could if they had "open" classrooms.
But the "fun" benefit was that if any of the teachers using that lab each day had requested apparatus/equipment set up for their lessons, our form teacher would "play" with it for us during morning registration - not every day obviously, but maybe 2 or 3 days per week. He'd not demonstrate how it "should be" used for the formal lesson, he'd do more entertaining things with it as he had the time to waste with it not being a proper lesson, only 5 minutes or so, but it was sometimes fun, sometimes thought provoking, etc., and it really gave me a better interest in Physics which I eventually took through to A Level (outside school).
I suppose it was a "lite"/safe version of the Brainiac science program (Richard Hammond's).
It really doesn't need to be "big" things or take a lot of time to bring a bit of life and interest into the classroom.