I like to be prepared for life's curve balls as much as possible. What mother doesn't? So I keep my minivan stocked with handy items.
If you have small children that throw up when they get carsick, then you will understand why I keep a portable carseat in my van. We were blessed to find one at a Walmart four hours from our house after our toddler (#1) had covered his entire carseat, it reeked, and there was no way to clean it or survive six plus hours of driving with it. We've used it more than once. It is easy to store and far better than having to buy a full size carseat. It also is easy to put in a friends car. Paper towels and a large plastic garbage bag are tucked under a seat for such times as well. We keep empty five gallon ice cream buckets stacked and at the ready too.
The potty. Anyone who's had to take a toddler into a gas station bathroom will understand this one too. There are plenty of portable potties on the market. I had purchased a folding potty adapter and really wasn't crazy about how it worked on full size toilets (if you ever get one avoid those that fold into quarters, they can pinch!), but it balances well on top of a five gallon bucket and we keep the bucket lid handy too. It's easy to take inside and dump that way. A packet of flushable wipes is stashed in the back with it on trips.
I keep a small diaper bag with a change of clothes for each child. This comes in handy for all manner of accidents as well as impromptu sleepovers or water play. It also contains small snacks that won't melt in the heat of the car.
The doorwell on the passenger side is stocked with extra diapers, diaper disposal bags, a changing pad, and a spray bottle of sunscreen. When I only had two, I could also tuck the spare outfits into a ziploc bag and stash it with them. I keep one of those round wet wipe containers in a spot in the console. I've found that they don't dry out in the car like the boxes do.
The glove compartment holds the boo boo box (one of those small bandaid boxes) stocked with child friendly bandaids (especially knee bandaids, am I the only one that finds regular bandaids to be too small for most serious scrapes?) I also keep a couple of claritin, Anna Kate's prescription, travel pills, and sting kill swabs. The latter is the absolute best treatment for stings. My grandfather and mother used them on me when I was a kid and I search high and low to keep them stocked at home and in the car. They numb the skin instantly.
We like picnics. I don't like germs and dirty picnic tables. In order to be prepared for a picnic in little time, I try to keep a picnic pack in the van consisting of a fabric tablecloth, tablecloth weights, napkins, plates, disinfectant wipes, and utensils. I keep straws and cups in the van too. They come in handy when the need for a drink strikes. I can use the drink cooler I often bring along, or buy a larger (cheaper) beverage and share it. Paper cups are also great for distributing snacks like trail mix, crackers, or, yes, m&ms.
In the winter a small blanket for each child stays in the car, and in the summer I often keep the beach bag stocked in there with suits and towels and a few dollars cash.
And for fun, we keep the car dogs, stuffed dogs that can used as friends or pillows, and if they get carted into the store, I can take them home and wash them without them being missed at bedtime. We also keep a frisbee and sidewalk chalk in the back, and kites tucked under the bench seats.
Of course we also have the usual maps, tissues (I do keep a packet of tissues in the backseat too to avoid the joy of trying to throw a tissue to the child in the back while driving), jumper cables, etc. But it's amazing how a little tucking and stashing can save a load of trouble later. And that works for me!