I always carry at least two spare bulbs of every bulb. Not so much for emergencies as for easy and quickness of replacement. A couple of times I have noticed a tail light out when at the shipper or consignee, and I've replaced it right there. Whenever I replace a bulb, I never toss the packaging until I've bought a replacement.
A spare serpentine belt should be carried, if for no other reason, in most cases, you can order it online much cheaper than you will get it at a dealer or repair shop.
I carry a gallon (or less if I've used any of it) of coolant. Mine's undiluted, so I also carry a quart or two of distilled water. If I need more than that, it's a more serious problem than I could solve by carrying extra coolant.
I carry a plethora of fuses and one each of the type of relays that will strand me on the road (starter relay, couple of others). Likewise a couple of critical sensors that will stand me, such as the intake air temp sensor. Cheap and easy to replace on the side of the road.
I carry at least one spare of every filter, oil, air, cabin and fuel. Again, mainly for ease of replacement. I carry extra oil, but that's because I change my own. It takes 9.5 quarts, but I don't always carry 9.5 with me. After I do an oil change, randomly, when I pass by an AutoZone, I run in and grab three quarts, which costs more than $20, and each $20 purchase is one-point ($20 or $200, it's just one point per purchase), and when I get 5-points I get a $20 credit that be used on any future purchase. So, I don't buy oil, windshield wipers and headlight bulbs all at the same time. I buy everything as close to $20 chunks at a time as I can.