Anything can become a projectile in a collision. The heavier it is, the more dangerous. I totally agree about securing "trunk" items in a hatchback/wagon.
And up front and in the backseat I try to ensure that nothing heavier than a piece of clothing or a stuffed animal is sitting out openly. I put stuff on the floorboards mainly, and keep a canvas bag in the car for temporary holding space for things like wipes containers, foodstuff, etc. I do have a kleenex box that sits on the passenger seat but I watched Mythbusters and they proved a kleenex box is not deadly.
So that's my one exception.
I would NOT leave something like a cellphone or a Sigg bottle sitting openly. My cellphone is clipped to my purse, and my purse is typically on the floorboard or wedged between me and my door. I tend to wedge things a bit -- like put the water bottle in the cup space in between my seat and the front passenger seat, then my sweater or the sling tucked over it. That sort of thing. Do those things protect completely? Probably not, but better than nothing at all.
I have a small car though, so the floorboard trick works really well. And things in the front seat would have a hard time maneuvering to the back.
The safest thing would be to have nothing loose in the car at all. That's unrealistic for me. So I try to minimize the heavier, albeit often smaller, items and ensure they're wedged or covered or strapped to something, etc., or shoved down in a bag on the floorboard.