The serious "must packs", other than the things I mentioned earlier are:
1. Power strip
2. ATM card
3. Additional Visa Credit card, issued from another bank and has different acct. #
4. Some gallon size ziploc bags for storage of messy stuff, spill-ables or lunches.
5. Leatherman multi-tool.
I always divide all credit cards, money and ATM cards between my wife and I. This is so in the event that one of us is pick pocketed or loses a wallet then we don't lose all money or credit card. This is why having Visa cards with DIFFERENT account numbers is important. If the account numbers are the same, and you cancel it, then you are both out of luck until the replacement arrives. This can be real pain in the rear if you are in a remote part of the world.
American Express cards have different account numbers but are not as widely accepted.
The ATM card is one of the best things there is: Yes, you may pay a fee for using another banks ATM but you are no longer required to carry large amounts of cash or traveler checks between destinations, or hassle with currency exchanges. If you have an account that limits or does not charge for using another bank's ATM, it is a great alternative. Check with your bank for accounts that have these options. You may, and probably will, pay a fee for using the ATM but won't have a second fee added on. I have Wells Fargo and the most I have been charged with my PMI account is $5 for withdrawing cash in China, Spain, Korea or England.
Just use a bank ATM when withdrawing cash. Try and stay away from grocery store type ATM's, especially, in foreign countries. Bank ATM's are plentiful almost anywhere in the world, especially near cruise ports.