I started with just buying runners, and I used runners as walking shoes in my city. I have now moved on to trail walking shoes because of the better tread and they are waterproof.
I recently purchased a new jacket for the spring-fall months. It's wind-resistant, water-resistant, and breathable. It has zippered pockets; it’s more for peace of mind knowing my stuff's not going to fall out. I don't wear this jacket in the winter months.
I purchased the Day Tripper Lite from Adventure Medical Kits Mountain Series. I purchased this from the Mountain Equipment Co-op (MEC) here in Canada.
I managed to fit a 13-inch laptop, water bottle, sweatshirt, and some large headphones. No expert folding skills here. There are some other smaller items that may have fit in the bag towards the bottom, but I just wanted to get a ballpark kind of idea for you. Of course, not all 15-liter backpacks are made the same, this is just an example.
I carry a typical water bottle of about 24 oz on my walking trips. Most of my walking trips are anywhere from one hour to 4 hours with breaks, and I'm usually in an urban setting or around an urban environment. So, I have access to water almost all the time. Sometimes I carry the water bottle in my hand, or I just place it in my backpack.
I've had a variety of hats over the years. The one I recently purchased was from the Mountain Warehouse. It’s a typical baseball-looking hat.