I have secure places to keep my bike at home and work. Last night I locked my bike up in public for the first time in a long time and was worried about what to take off and what to leave. I ended up putting my lights, bike computer, pump, and spare tube in my packpack and left the pannier attached.
For quick trips into a store or restaurant, what do people take and leave? Obviously it's safest to take everything, but sometimes difficult, especially if you aren't carrying a bag or a jacket with deep pockets.
I see a lot of bikes downtown with missing seats. Are seats that much of a theft target that people carry them in? What else should I defend against getting stripped? I use a U-bolt with large cable through the rear tire. My bike is a Trek 7100 so lower end parts. How do my worries change with a more expensive bike?
I don't have anything on my bike that can't be taken off without using a wrench and I haven't had a problem. I lock up my helmet by putting the strap through the U-Lock. For carrying things I put a basket on my bike to just grab my bag and go. I did invest in a Kryptonite NYC lock that I use to lock both my frame and rear wheel so I don't need to lug around a cable. My bike isn't so very expensive but I love it so why risk it with a cheap lock? My rear lights are cheapo/freebies so I just leave them on but make sure to turn them off so they aren't beacons screaming "steal me!" to drunkies walking down the street.
I always leave my lights and my first aid/spare kit attached to the bike, along with my emergency tire pump. Haven't ever had an issue in four years, until Tuesday, when someone stole my bike pump. Looks like they tried to take the first aid kit, too, but couldn't figure out how to get it off. Left all six lights, though, and didn't mess with the bike otherwise. Nuisance crime is so inexplicable.