I once took Benadryl for allergies, not knowing of its side effects. I took it on my way to a full eight hours of sitting in an overly warm room (it was on the second floor of an office building with a broken AC and an ancient projector was running in the middle of the room ... In August, IIRC) listening to someone lecture 20ish bored people about how to do our then-new job. The best I can say of the day is that I managed to stay technically awake.

I'm pretty sure that was also the last time I took Benadryl. I would not describe the experience as enjoyable. I've taken similar medications since then and have appreciated their efficacy and aid in getting to sleep, but I can't imagine taking them recreationally. No judgement to anyone who does, it's just not something I can interpret as fun.

edit: Though I never saw the Hat Man (unless he was incognito as the lecturer). Maybe that's the fun part.