Some companies offer compact reusable bags that can be stored in a coat pocket. For example:
https://seatosummit.com/products/ultra-sil-day-pack - I've had a few different versions of this bag for years. There are also cheaper/bigger/different versions of the same sort of thing you can find online for "packable daypack".
https://nanobag.com/products/nanobag - I have heard good things about these, but I prefer a backpack because it allows me to be hands-free, or to use my hands to hold more items.
However, I would start by carrying a lightweight "single-use" plastic bag, and simply re-use it. Plastic bags are not as strong as these premium bags, but they hold up well enough to be useful in most scenarios.
Thank you, these look amazing, I'm going to get one! They don't entirely solve my problem because they don't get large enough (for the smaller sizes, the larger sizes are too large when folded) but they'll be useful to have.
https://seatosummit.com/products/ultra-sil-day-pack - I've had a few different versions of this bag for years. There are also cheaper/bigger/different versions of the same sort of thing you can find online for "packable daypack".
https://nanobag.com/products/nanobag - I have heard good things about these, but I prefer a backpack because it allows me to be hands-free, or to use my hands to hold more items.
However, I would start by carrying a lightweight "single-use" plastic bag, and simply re-use it. Plastic bags are not as strong as these premium bags, but they hold up well enough to be useful in most scenarios.