I usually take what I would take anywhere - so that is wallet, phone, leatherman, paracetamol, anti-histamine, sunscreen (if required) etc. On top of that, if I can walk home or leave the bike somewhere safe and get home, then not much else. Otherwise, toolkit, lock, spare tube, pump, drink, snack-food. I usually carry an ICE card too. The toolkit is a roll with puncture repair kit & levers, allen keys, a couple of spanners, chainlink tool, spare cables, nuts, bolts, washers, electrical tape, cable-ties, duct tape (wrapped around an old plastic membership card), all rolled up with a couple of toe straps around it (handy for all sorts of stuff including 'third hand' stuff). When the weather isn't brilliant, something warm to put on in case I have to stop for some time. If rain looks likely, waterproofs. Sometimes a small first aid kit including a small tin of vaseline.
I don't have recourse to a phone-for-backup/rescue car and so I like to feel a bit more prepared than most might hence all the kit. For day rides, I usually have a rack-top bag and a handlebar bag (as in my avatar) and I'm not one to fret about the weight.