Back when I had a vehicle (we shall meet again Blue Lady, one day) I always had a bag in the back. In the summer it was full of old shorts, a t-shirt, flip-flops, and a blanket. In the winter some gloves, a hat, old hiking boots and my Carhartt coveralls just in case. Depending on the size of your vehicle, it’s not a bad idea to have some extras.
When the weather is warm it’s almost easiest to keep a car-camping kit in the back. It’s a much lighter load than in cold weather, and can be a life-saver. Impromptu trips to the lake, coast, mountains, or anywhere outside the city require zero planning. Outside Online has their usual overpriced gear, but you can find cheaper stuff at local sporting goods stores.
I will say it never hurts to keep a swimsuit, flip-flops and a towel around. They’re cheap to buy, take up hardly any space, and can be a great investment in the summer. Hell, the winter too if you’re into the Polar Bear Club or just insane.