Regarding bags, for general casual street use I use a light Samsonite messenger bag with a flap that closes over the top of all the zipped compartments for security. Its main compartment is just big enough for my reading tablet (in a sleeve), my street camera, lunch and drink, and a jersey if needed.
An arm is always looped through the strap no matter what and I have never lost anything. But nearly did in a Paris restaurant (one of the less salubrious ones with bench seats and tables) but was very lucky; I left distracted by talking with a friend and some wine
, and after walking 5 minutes or so realized I had left my bag behind with wallet, a camera, etc. in it. Rushed back and the proprietor met me with a big smile; the next person who sat in the same place was a very honest man who handed it in (he was in fact the main male actor in an American TV series of the time).