Generally on my local trails where I may ride for less than a couple of hours I take a small water bottle and my repair bits (multitool w/a chain breaker, dynaplugs, CO2, tire lever, rotor truing tool, spare chain/power link) and my car keys in a Race Face Rip Strip. Phone and food goes in my If I am heading out for more than two hours or it’s exceedingly hot (like most of the summer in the ATL), I will schlepp a Source hydration fanny pack. Finally, for the big all day rides my monster truck EVOC pack comes along for when I need to take my life with me.
I use a Revelate Jerry Can to carry tube, levers, multitool, quick link, and first aid. Water bottle and pump are on the down tube. When I need to carry more stuff like extra water, snacks, rain jacket, etc. I add an Oveja Negra Chuck Bucket and/or Dakine Hot Laps. I do whatever I can to avoid carrying a full backpack.
I use a Revelate Tangle bag on my fat bike. Best investment ever. I can store tools, layers, or a full 2 liter water bladder in it while still having access to the lower triangle of my bike for bottles or whatever else I can fit on the water bottle bosses. I would love to run a custom full frame bag, but the tangle nicely swaps between all of the bikes in my stable.