I'm working on carrying even less in my BAG when I'm not using the car.
I found that coins and the camera are the heaviest things in my bag (must figure out if it's worth having a travel card if I'm going to the city). A smart phone with a decent camera function would be good, or when the time comes for a new camera I will look for the lightest. A single paperback is OK, magazines are very heavy but can be left or given away at the end of the journey, and you can just tear out any pages you want to keep. A Kindle is obviously the thing if you want to read a lot. Food is heavy - I try to buy as I go - the easiest way to carry it is inside me! A bottle of water is very heavy. I've found a very small bottle and top it up when I can, or just buy a new one, or ask for a glass of tap water when I order a coffee and drink it then.
I've taken to using a little (child size) back pack. Canvas or nylon are lightest and I'm on the look out for a really nice one. I like having my hands free though with a back pack you must beware of pickpockets - zipped pockets well down inside of the bag are good.
Here's my list of what it contains for a few days away:
Purse with cash and cards (only the cards I expect to use - every little counts)
Keys, including key for padlock on suitcase
Mobile (charger in suitcase)
Pen and paper
Camera (charger in suitcase)
Short sleeved t shirt or vest top (to substitute for the one I'm wearing if I'm too warm, to add underneath If I'm not warm enough
Scarf, if I'm not wearing one
Tiny packet of wipes
Su doku or crossword cut from newspaper if it's a long journey
Do you have any other must haves in your bag?