Hm, quite a few! I've also been pretty lucky in what I have done, so it isn't like I'm unhappy or anything though.

Buuut, probably the easiest would be watersports with a pussy-haver. The opportunity has just never come up!

A bit more difficult would be some long term conditioning-type stuff. Maybe cumming on command, or associating a particular scent with sex. I dunno, something like that. I've done a bit on my own, but someone else dictating the terms... Hot.

Being a party favor would rank pretty highly. Passed around and fucked, maybe blindfolded, etc. This one'll probably stay a fantasy tho...