It seems counterintuitive to try to plan ahead for an unexpected situation: you don't expect the situation! That's literally the whole point! But, surprise situations arise all the time, and thinking they won't happen that day has always led me to zero waste slip-ups. A quick walk turns into a longer one, you happen to be near a farmers' market and decide to drop by, you get hungrier than you thought or your transportation gets delayed or your first-choice restaurant is closed and you have to wander down the block to someplace else.
I've found that no matter what I think I'll be up to that day, I'm much better served if I carry a few things with me that can be helpful when my plans change. To wit: today we'll be expecting the unexpected. Here are a few of the tools that help me stay flexible and zero waste at the same time:
Insulated water bottle. A water bottle is a clear winner here - but if you choose to bring an insulated one instead of a regular bottle, it can also serve for coffee and tea, too. I like to use a Klean Kanteen (I have this one), but there are tons of options out there. (You can find more ideas of simple, cheap options for grabbing coffee and tea on the go, here).
Tiny utensil. A small, lightweight one like this or this takes up so little room and can give you the freedom to eat when and wherever you'd like, no worrying about plastic utensils needed. Or, you can offer it up to a friend who's without one that day.
Cloth napkin. Such a versatile thing! I usually fold mine up around my utensil in it to keep said utensil clean - a few friends roll their napkin around several utensils and secure the bundle with a rubber band. From there, you can find so many ways to use the set. This weekend, I tied my napkin in a bundle to hold apples from the farmers' market. Here's a primer on tying fabric squares, like napkins, into versatile, useful shapes, furoshiki-style.
Small container. This could be a jar, a little glass or metal container, or even a cloth produce bag or Stasher bag. Any of these can be useful for holding compost generated throughout the day (apple cores, tea bags, etc.), but I'm partial to carrying one of the first two options, which can also hold restaurant leftovers if needed.
I mix and match the above depending on what I think I'm most likely to need that day (also: what I can fit in my current purse of choice). Each item on the list is hardworking and versatile, meaning I'm typically covered no matter the situation if I have a few of those on hand. In a pinch, I've used a clean handkerchief (full disclosure: also a not-so-clean one) as a napkin, my water bottle to hold a banana peel to compost later, and a mason jar intended for leftovers as an impromptu water glass.
A few other things you could consider keeping on hand: a piece of fruit or a little bag of bulk snacks, to ward off any thoughts of vending machines or other packaged snacks. A metal straw, if you're fond of using one when out and about.
Your list of daily go-tos might look different, and if so, I'd love to hear: what do you find most helpful to carry with you?