While not technically bike gear, hydration is an essential part of riding. A while ago, I purchased a pork chop (small frame bag) for my Ripley, so I had a place to a spare tube and the usual flat-fixing gear. That enabled me to carry a small Camelbak, which I used without the bladder, for carrying snacks, first aid kit, and ID. I started on these scouting rider with one water bottle. I soon added a second one, stuffed into the Camelbak. As the rides got more remote, I put the bladder back in the Camelbak.
I do not take the trailer with me when scouting, and I soon found that I needed to carry more food, back up equipment (lights, Mophie charger for the phone), and clothes. After a bit of research, I settled on the Osprey Raptor 14. Awesome pack. It has about a hundred pockets and easily carries everything I need. It’s not perfect—what is—but it goes a long way to make me feel that I heve just about everything I’d need when riding in more remote areas.