This might sound a bit odd and unexpected but the hottest thing I've seen my wife do is get into some deep passionate kissing with the others she's been with and here's why:
First time we made a plan to hook up with a guy we each established some boundaries we felt shouldn't be broken and shared some non-negotiable rules with him so he knew what the boundaries were. Some of the rules were ours (no hitting, slapping, pain or humiliation of any kind we were there to have fun) but of course she had more rules than I did, one of which she was adamant about was no kissing. Might seem strange that we were going have sex with someone and kissing of all things was off the table, but one of the "what if hurdles" she had to get over was - what if I develop feelings for the/a guy - and she felt that kissing was more intimate for her than sex for sex's sake and likely to lead down that path so okay, no kissing.
First time, the three of us are together we land in the hotel room we booked and things started out exactly to plan; after soaking in the whirlpool tub in the room the three of us land in the bed and for a little more than an hour the three of us of us mixed it up various ways, all rules were followed and it was a blast. After a second dip in the tub to rinse off, the three of us were sitting in the living area of the suite, the two of them next to each other on the sofa, me in the chair across from them while we enjoyed a couple drinks and chatted about just stuff. Suddenly and without warning my wife leans over and starts kissing the guy, I mean deep, long, tongues slopping around each other's mouth kissing for a good ten minutes - instant boner! Eventually she stands up, opens her robe, climbs on him cowgirl, guides his dick in her and starts riding him while resuming the passionate kissing (picture in gallery).
In the moment I realized that by breaking the first of her own rules any doubts she may have had about her decision to go through with this and any I had about whether she was feeling any regret were washed away. I knew she was enjoying herself by virtue of the fact she was not only kissing him but how she was. Second rule she'd insisted on was that I participate in everything, again part of her "what if protections" her reasoning being my presence would be a constant reminder it was WE doing it and not just HER and she tossed that one aside as well with this move. They fucked on the couch in various positions for a while as I watched, took pictures and video until he came in her again. Then after another rest and some hydration back to the bed where they kept up with deep kissing and fucked until he came a third time.
I've watched her do a lot of hot sexual stuff over the years but nothing is quite as hot as watching her passionately kiss her partners because I know it means she's really into what she's doing and that means everything.