Quite a lot of things. Though one particular of note is that I enjoy being deprived of air by my girlfriend. Only when done carefully, it's something I've only ever done with this one particular person due to excessive trust in her being careful. Never intentionally brought to a point of outright unconsciousness, but ideally to the point where I am dazed and just barely still conscious.
It enhanced sensations for me, and it makes me highly susceptible to suggestion. It does wonders for getting me immersed if we're role playing since, if done right, it becomes a bit more difficult to separate reality from fantasy (albeit, I'm not exactly in a state to be actively participating). Plus it just gives my girlfriend free reign to do whatever she wants to me with little resistance, which seeing as I am submissive to her being dominant, just generally makes things delightful.
Of course I want to emphasize that we are always very careful when doing this, almost always done by hand or with implements that pose no risk of being stuck or otherwise having difficulty being immediately taken away.