Hi Rex,
Guests use all kinds of bags, and I've always liked small back packs. However, I've become partial to using a diaper bag (don't tell my kids!). I like all the side compartments to hold all the little things rather than their getting lost within the big open spot of a back pack, and diaper bags today certainly don't look like diaper bags.
These are the things I bring with us:
small tubes of sun screen
sunglasses (and, ug, reading glasses for me)
some snacks like dried fruit
a little "first aid" bag: bandages, Blister Block, and Tylenol
hand sanitizer
chap stick or lip gloss for me :-)
a camera with extra battery, memory stick, film etc. and a large Ziploc bag in case of rain or a water ride
cell phone (our children all have our cell numbers memorized or written down)
driver's license and resort id
a debit/credit card along with a small amount of cash
some loose change for making wishes and penny presses
our trip itinerary with all dining reservation numbers
Disney pins (my sons trade pins, but they don't use the lanyards)
an umbrella and rain ponchos (depending on the season)
I do keep my "tote" bag with me on all the rides. Typically, it fits at my feet while some rides have little storage nets for personal belongings.
I hope this helps with your planning - Kara