When I was a horny pubescent 12 year old — markers. I even tried using those toys that were fashionable at the time – they were squishy and filled with gel but resembled dildos (in my mind). Obvi didn’t work out well. Also experimented with the vibrate setting on a cellphone. Certainly humped a few pillows.
N64 controller with a Rumble Pak. I just walked Mario into a corner and rumbled away.
It was the only thing readily available that could pass undetected in my room when I was a teenager.