Depending on how much it being relatively covert changes things, this is a toss-up for me.

Got a blowjob start to finish sitting in the bleachers at a baseball field from an ex. First time she'd ever given head, but between enthusiasm and apparently enjoying the public setting it was very fun.

Then there was one dinner out with family where my SO was wearing a remote vibe. I had the remote. We had fun playing with it all evening, keeping it relatively steady but not -quite- obvious to anyone what we were doing. That paid dividends when we got home: apparently she liked public play too.

Me and my at the time girlfriend met up at the park for a picnic we planned. I had been out of town for work for a few months so we were very happy to see each other but also very horny.

We relax on the blanket together cuddling and she slides her hand down my pants and fondles me. I return the favor.

Without a word being exchanged we get into a 69 position and go wild until we both cum about 3 times.

We were noisy and it would have been very easy to be spotted but surprisingly we weren't.