My husband would say that I don't have just one thing that I have to do before going to bed. I check all the locks after letting the dog out the last time. I check on my children. I have a routine of getting a glass of water, checking the baby monitors, going to the bathroom, putting lotion on my hands (I can't go to sleep with dry hands), putting on chapstick, turning off the lights, saying prayers, and drifting off to sleep. I have to do all of them or I cannot go to sleep (I wash my face and brush my teeth earlier in the evening).
On the other hand, my husband takes off his socks and pants next to the bed and climbs into bed. He's asleep almost instantly. We're so very different!