A small first aid kit and and a tiny sewing kit are normally not needed. If the need arises they are invaluable and take up very little space. I also pack a lightweight foot & ankle brace and arnica montana, because I do a lot of hiking and occasionally have a mishap.
My Rick Steves toiletries bag holds all the shampoo, lotion, sun block, laundry soap and tooth care I need and then some. I don't use up all the stuff in that bag, but what's left over isn't a burden.
Unless I'm going to a tropical country, I almost always pack my lightweight Goretex jacket, whether planning a hike in the mountains or city sightseeing. I usually don't care if I get wet in a light rain, so most of the time I don't need it, but was happy to have it for several rainy days in Paris. Another time was in the snowy mountains above Chamonix. It served as a windbreaker layered over a long sleeved shirt and a sweatshirt, which was adequate because I was in motion.
I also carry either a water bottle or hydration pack (the latter doing dual service as a day pack). Like several others have posted, I refuse to buy water in plastic bottles unless it is an emergency. The minor inconvenience of traveling with a water bottle is nothing compared to the damage plastic is wreaking on the environment! I sometimes also pack a filter straw for questionable water.
The one time I under-packed was for a trip through Spain and Portugal with my daughter and her two small children. We were models of traveling light, but it backfired when the kids got their clothes messy (REALLY messy) and we had to do more hand laundry than anticipated, or the time the three year old went head first into a fountain. We bought him a nice dry souvenir t-shirt in a nearby shop. Seriously, a few more small shirts and shorts and some extra socks wouldn't have killed us!