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.
If the area doesn't look very "lively", sometimes I just leave my rig outside a shop. My little trick though is I put my bike into some nasty gear, like either the fastest one or smallest/smallest ring combo, so the chain hangs loose. Or even I tune my front derailleur in a way allowing me to get the chain completely off the front chainring (in a way which requires stronger than normal push on the shifter). So in case of emergency I'd hopefully have a couple seconds more to chase the thief.