I use a small crossbody purse that holds a surprisingly large amount of stuff. I always have a couple snacks, a small breath mints tin from Whole Foods full of stuff (pill pouch ziplock bags each stuffed separately with Tylenol, bandaids, mints, & meds), my wallet, a small lanyard pouch with AP & fastpasses, a small notebook, a couple of pens, an inexpensive rain poncho pouch, backup glasses, phone & charger battery pack. I also often have a small sandwich as well & I have a foldable plastic water pouch which I clip onto the outside of the purse. It works great & holds what I need.
I often travel solo in the parks, so I have to take my bags on rides with me, so I like something small enough to easily fit into the security pouch in Indy or BTMRR so I do not have to worry about losing my belongings. I also physically cannot do backpacks due to strap placement always pressing right into my implantable cardioverter defibrillator (like a pacemaker, but more high tech) & I find them terribly uncomfortable. The only way I can do a backpack is if I put it only on the one shoulder without the ICD, and that always leads to neck and back pain. I think my system works pretty well for me.
~Meg