I think you will find many threads that deal exactly with this subject.
For all trips I take a
Basic first aid kit (tylenol, bandaids, tweezers, clippers, tums, dramamine, safety pins, small sewing kit, etc)
Power strip for charging phones, camera batteries, etc
Walking Shoes that are broken in
Sunscreen
Umbrella/ponchos - something for rainy weather
Cross-chest purse (or fannypack) ... just something to keep ID, tickets, etc where you know where it is and won't need to be "set down" and forgotten
Small backpack
Camera
Hat
Sunglasses
Paperwork including receipts, reservation info, itinerary, etc.
MONEY (including small bills) and credit cards
Go through wallet and leave all unnecessary papers/cards at home (library card, etc); photocopy what you are taking and leave a copy at home and take a copy with you, storing separately from originals
Key in your cell phone all important phone numbers: Doctor, dentist, CC company, bank, etc
If you are renting a car, take a copy of your auto proof of insurance
If you are flying, take a picture of any checked luggage.
If you have small children, take a picture of them each morning with the camera (probably wouldn't ever need this, but, hey, good idea if they ever wandered off and you needed to remember what they were wearing, right?)
Well, this is a start ... but just run through a typical day in your mind and put items down on paper.