Where do you leave your bike on cafe stops and water breaks? Where should I leave my bike on longer rides when stopping at a cafe or going into a store to get water?  Do you carry a lock in your jersey? Leave it outside?  What is a good lock?  What should I do so my bike doesn't get stolen?
There is such a range of answers.

A lock mounted to the bike is going to be more comfortable.   

A decent light ulock is going to be 1.5 lbs.  

A cable lock is going to be more like 0.5 lbs, but not as strong. 
But, for a quick in and out it that may be the best trade off.

The weight of the bike plus the weight of the lock to protect the bike is typically a wash. 

In a group you could have one lock and a cable, or just have one person at a time stay out with the bikes.  Alternately, you could just have the cheapest bike in the group.

Some establishments may let you bring the bike in.   

If you can see the door then you can remove a wheel and bring it with you. I will loop my helmet through the rear wheel. Not much but you pick 5 seconds. You can run down someone carrying a bike.