I have to be ready to respond to work emergencies, so have to keep some tech handy: iPad, WiFi hotspot, batteries, aspririn.
Also, our kids have food allergies, so we have epi-pens in Frio cases to keep them cool. Until this year my wife and I took turns shlepping those, think we'll have them keep them in their own packs this year. (But, will probably carry an extra set too.)
Otherwise: camera on my belt, in bag: ponchos, home made allergy-friendly trail mix, first aid stuff (moleskin, bandaids etc), sun block (I'm almost invisible and kids inherited it), baseball cap, Frogg Toggs, backup set of touring plans printed.