A common approach is to lock the bike to something solid and to keep all your valuables like wallet and passport etc. in a handle bar bag that you always take with you. Don't worry about stuff you leave on the bike, 99.99% of the time it will be there when you get back, and if it isn't you'll have to get by without it until it can be replaced.
That is what I do. Handlebar bag is quick release, I can put on the shoulder strap and be on my way in half a minute.
I have only left the bike at restaurants, bike shops, grocery stores, hardware stores, etc. At these places, most people are in and out in less than an hour. I would be more concerned at other places where most people would be there longer than that, such as museums, etc.
Restaurants, I often use a small skier lock on the bike instead of digging out the bigger lock. Such a lock only slows down a thief, but at a restaurant I always try to sit where I can see the bike. Thus, slowing someone down is all I need. But grocery stores I use a really good lock.