DS has a "busy bag" in my trunk that I use for restaurants, church, Drs appts, etc. I rotate new stuff in about every 3-4 months. He's 2.5 and right now it's teagu blocks, small coloring pad and crayons, several matchbox cars, and a tow truck.
I normally keep a diaper changing mat, etc., in my (large) purse. So it's not often I use a diaper bag.
There's usually a book or plastic car in my purse. In the car we keep a touch/feel book, a couple other random toys. There's also a large ball and picnic blanket in the trunk of my SUV, along with a small bin in both the trunk and backseat with diapers, wipes, a change of clothes, and snacks.
Off topic - but I am really thankful we are over-prepared. DS got car sick earlier this summer, and we were able to pull over, open the waterproof picnic blanket open in the trunk, set him down, strip him, wipe clean, change, throw on new clothes, tackle the car seat cleanup, and carry on with our drive. Had we not had backup supplies we would have been TOAST.