In looking at what others recommend, it's really easy to go overboard and haul along all sorts of stuff. I used to be that way. Maybe that's why Pearl weighed exactly 2,000 pounds on Danny P's 4-wheel scales. 
Since those fatter days, I have cut back a lot but have a couple of small tool bags and a tool roll-up that, between them, hold a number of "multi-tools" and a select assortment that'll be handy off on the side of the road. Honestly, I've used my road tools on other people's cars a LOT more than on mine, but that's why I bring them.
On @Jack Crosby's trash bag idea, he carries it to put the road tire in when using his spare tire/wheel. The road wheel won't fit under the hood and putting it in the trash bag keeps Alice from getting all grubby when it's riding on her lap to a garage. 
And, of course, I have my Hagerty roadside assistance number on speed-dial.