I have random stuff in the back , clothes to go to goodwill that I haven't taken yet, shoes, some random coats, bag of stuff, my net for collecting bugs, and a stick to prop it open. I really don't check all that often.
Our trunk is fairly messy but it has jumper cables, a roadside emergency kit, an air compressor, a bag with winter clothes (hat, scarves, gloves, etc.) for when we need to put tire chains on the car, tire chains, and a bag of kitty litter for when we get stuck in the snow.
For the most part my car trunk its always clean, with everything put away, because i use it often when im traveling to my partners - but for almost a year i had a couple of Slut Walk signs my friends and i had made for the event just hanging out.
Trunks are great for collecting all the odds and ends that get left in the car but never quite make it back in the house I normally have a disaster kit, umbrella, change of clothes, sweatshirt, emergency cash, reusable shopping bags, and stuff that I forgot I left there.
My trunk is currently a mess! I always keep a spare tire, jumper cables, a blanket, shopping bags, and car cleaning supplies. At the moment there is also an old printer and broken air humidifier (have to recycle them or something), a box with a bunch of rand crap in it, and paperwork. Definitely need to clean it out!