Breath play can be fun, esp with Dean and Cas. I’ve read several fun destiel breath play fics. mmmm.
My favorites are totally dependant on what version of these characters we’re talkin’ about. Like, if it’s angel!Cas I have a huge thing about Dean tugging on Cas’s wings and having it drive him absolutely crazy. Also, Cas glowing and causing lights to shatter in whatever room they’re in when he comes. Or like, Dean realizing partway through that he’s essentially having sex with a being that’s more force of nature than human and kind of getting off on the idea that Castiel could literally do anything to him (maybe a bit of fear kink in there).
If we’re talking human!Cas I love the idea of their sex life starting out really timid. Like, every act is desperate in a way because they’re both overwhelmed and excited and happy but they can’t get their hands to stop shaking and they have to go slow or they’re going to wind up frantic and it’s all gonna be over way too fast. Obviously, as they get used to each other they mellow out, but I can see the first time or two being timid in a way. (But I’m a sucker for that kind of shit so that’s just me.)
Also, I like the idea that they’re kind of exhibitionists. Like, maybe the first time it’s right after a hunt and the adrenaline is racing so they just kinda… screw in a graveyard. And then Dean makes some comment about them being the envy of every Byronic teenage boy and Cas is like “do what now?” because Dean’s definitely going to admit to that super brief time when he was fifteen and thought it might be cool impress goth girls and so he read a lot of Romantic literature and wore eyeliner that one time. (But that’s a headcanon for another day. Anyway…) But Cas kinda likes the risk and Dean’s got a bit of a thrill seeking streak in him so they wind up trying to tease each other and get each other off in places they really shouldn’t be having sex.