Blowing someone when he's just a head, them resting his head between their legs. Being fucked while his arms and legs are missing. Sucking his own robo-dick or his partner using it as a dildo. Whatever works. Genos is a cyborg so his limbs can be removed with (relative) ease and with no gore. Any level of consent works.

Either Genos being re-purposed as furniture - a human chair, a foot stool, a (sexy naked) lamp - or being put into storage as a form of bondage.

Mess with his programming and turn him into a sex toy, trick his body into experiencing arousal when he's not interested, or whatever. This is sort of a mind control option but for a robot dude, only his brain is human so his thoughts can't really be changed too much. Anything that falls into that category is cool.

Genos' mouth is a sensitive part of his body, pretty close to human, and it'd be great for his partner to explore it with their fingers, toes, a toothbrush, food or even just a big ol' dick.

Genos and his partner experimenting with his fancy cyborg body. What kind of features does it have? Plug and lay (heh) dick attachments spring to mind. But any weird and wacky robo-kink fits under here. Oh and he can fuck his partner for hours without getting tired, probably, so that might be fun.

Someone turned giant? Someone shrunken down somehow? Whatever, man!