I don't wear a sweatshirt when it's 60 degrees outside (some people do!), but you can be darn sure I am wearing a jacket or coat when it is 30 degrees. I live in SoCal -- have lived here my entire life -- and when it drops down to certain temperatures I have to grab some kind of warmth.
And I always have at least one bottle of water in my bag on a day trip -- there is a compartment for the bottle, right there on the side of the bag -- if not two. If I am staying at a hotel, I bring at least 12 bottles of water with me and load up the mini-fridge. I use every last one of them too -- nothing goes to waste. In my daily life, I really only drink water and black coffee. I might 'splurge' and drink a Coke or some juice every 3 months or whatever. So it makes sense for me to have a lot of water with me on DLR trips.
I find that I have an easier time spending money on food in the parks than I do on buying beverages. The beverage prices are crazy. If I want to buy a bottle of water or orange juice from one of the outdoor carts, I will be paying almost the same amount that I would be spending for a 12-pack or 24-pack of water bottles at the grocery store! That's just not right. I can justify the expense more if it is food -- or some sort of food item that I only eat at DLR and nowhere else.
Otherwise, I have to have a couple of different sunscreens with me at all times. I have to have a camera, back-up battery, extra memory cards, hairbrush, lip balm, money and ID, regular glasses and sunglasses, cell phone, Purell, breath mints or strips, Advil, Kleenex, Band-Aids, etc. It all adds up and takes up space in the bag. My bag has pockets for everything. I don't necessarily like carrying around a bag loaded down with stuff, but you can bet that I am glad that I have that stuff with me when I find that I need it. There have been many occasions when I have needed to pull something from my bag o' tricks -- an unexpected headache, an unexpected blister, my scalp suddenly getting sunburnt and in need of a spritz of sunscreen spray, mind-numbing pain shooting down my leg, etc. I am never sorry that I have these things with me.