Sleight of Hand
I have a game I play where I often try to think of erotic situations in mundane settings that my husband and I find ourselves in. It all started on a ride back home after a dinner party from the home of another couple, where my husband and the other man's wife, who have been platonic friends since high school, monopolized the conversation for a good chunk of the night.
My fantasy is where we're at a fancy dinner party at their house, but the dinner party is a ruse for the other woman's husband to seduce me. The other wife engages my husband in conversation, flirting with him to distract him from her husband's advances. My husband is oblivious while the other husband slides his hands up my leg, disappearing under my dress. I start to squirm and breath heavily, unnoticed by my husband as I am out of his field of view, his vision fimly on the other wife. The other wife exchanges knowing glances with her husband throughout the exchange.
Other times this happens at the kitchen table, where I am sitting next to my husband and the other couple are sitting next to each other, but the other man at some point relocates to the head of the table adjacent to me. In other versions we are having drinks in the living room and the other wife distracts my husband by pulling out an old yearbook and sitting beside him, or showing him their collection of vinyl records, whilst I'm seduced as he flips through them. At some point I go to use the washroom and the other husband follows me, unbeknownst to my husband who is too enthralled by the other woman, and I am seduced by the other man.