I like this topic!

There are a ton of different things that I love to do, and might describe as fun afterwards, but at the time I'm not thinking 'fun' so much as some other choice adjectives...

One thing that I can say for sure is lots of fun at the time, is when my masochist is showing big time, and he indulges me. We will sit around, and he'll grab a cane and whack me as hard as he can. Generally this is followed by a few curses and then giggles. I will laugh my ass off, then he does it again. Stingy pain does that, it makes me giggle. Or, more correctly, it makes me curse then giggle.

So that's my most fun activity...not necessarily my favourite, but that one that's FUN! Getting hit really hard with something viciously stingy.
...man, masochism is so much fun!