Everything! I leave nothing to chance. Some things require checking up to 15 times, like locks. My husband is double checking the admission papers for my funny farm debut.
Every night, I make sure my iPhone is recharging, and there's a little green box on the screen, meaning it's charging. But the first thing I do is ask my phone for a battery check.
I double check to reassure myself any sort of awkward, potentially precarious load of whatsits in my pickup-truck bed is very securely lashed or strapped in place.
Car door locks, any door locks
Turning off gas stove
Car keys in pocket (Mainly as the carabineer clip I use for a key ring is getting loose, so time to replace)
That my phone is either turned off or is set on "do not disturb" when going to bed for the night. If I get any text or phone call after I go to sleep, I will not be able to go back to sleep for 2-3 hours.