Documents: Please check for accuracy (spelling, etc). Put these documents in a safe place. Print off any vouchers you may need.
Flights: booked? seat selection? You can double check with your agent
Schedule changes: check about 72 hours prior to all flights to ensure there have been no schedule changes.
Travel Insurance: If you are relying on an extended health plan do you know if you have coverage limits or deductibles? Will they pay upfront or do you need to get reimbursed?
Online check-in when possible to make things smoother at the airport.
Baggage allowance: double check how much you can carry on your airline
Passport: Is your passport valid and undamaged? If you are travelling international is it valid for up to 6 months beyond your return? If you don’t have a valid passport do you have other government issued photo ID that will be accepted for check-in? Do you need any Visa’s for countries you are visiting? Do you need any vaccinations for the country(s) you are about to visit. This should be checked way in advance not a week before
Photocopy all documents and leave them with a friend or family member.
Register with your foreign affairs department when you leave.
Do you have prescriptions and are they in their original bottle?
Double check liquids and gels allowances on planes <link>
Have you made arrangements for your pets?
Is someone looking after your house? Many house insurance policies require someone to check on your house in the event of some sort of problem. Otherwise your insurance may not cover damage while you are away.
Sunglasses, hat, sunscreen
Glasses/contact lenses/extra pair
Travel first aid kit
Ziploc bags come in handy for a variety of reasons, make sure to pack a few
Comfortable shoes to walk/hike in
Ear plugs/sleeping mask/inflatable pillow
Many airlines now charge for food on a flight and most airlines will now only accept a major credit card and not cash or debit.
Remember that all hotels require a credit card at check-in and will often authorize a value of funds at check-in and not release these funds until check-out.
If you are renting a car you will need a major credit card at check-in and they will do credit hold on your card. Have you thought about car rental insurance? It might be better to buy the inclusive package instead of the insurances at the counter.