DH has a backpack he loves, it has some kind of vented/ cooling back. He uses it to carry some camera equipment (we're photographers who bring our DSLR cameras with us), some diapers/ wipes for DS2, and sunscreen, and our cheapo $1 plastic rain ponchos. We could load way more into it, but we don't. Actually on some rides DH has stuffed my entire purse in there as well and we still have room to spare.
I have an ebags Piazza daybag that fits more than I could have imagined and has compartments to keep it all tidy. Again, I don't load it with stuff but I could if I wanted to. It's 9"x9" and carries my DSLR (I added a camera bag insert to keep it cushioned when not in use), small change purse wallet (which holds ID, credit card, some cash/ change, and a few bandaids), trial size body powder (DS 11 chafes something fierce), some ibuprofen, autograph supplies if we're doing character meets (so, a few markers and a book of index cards), my phone, and sunglasses (when I'm not using them).