I also have a piece of masking tape on my sump with a magic marker writing that says "Sump Fill Level" with an arrow pointing to the bottom of the tape.
I leave extra saltwater at 35 ppm in two 55 gallon barrels in the side yard in case of catastrophic tank failure, they can throw the corals/fish into them.
I leave extra RO/DI water for top off to refill the reservoir or to manually top off the sump, as I have my top off set to slowly top off my tank.
I've also labeled all my cords. If you've ever looked under your tank, it's probably one confusing jumbled mess. If a pump goes off, starts to burn or cavitate, I want to be able to tell them on the phone which cord to unplug. I don't want them to have to plug and unplug everything looking for the correct cord.
We leave our house AC set at 76 degrees so that if there is a power failure my tanks are not running at the brink of warm and have some leeway if the house starts to heat up and the chiller is off because of a power failure.
I have my Ampmaster 3000 hooked up to a Smart UPS 2000 and it will run for 6-7 hours. We also have two generators that they can use.
I turn my finicky external skimmer down so that it will not overflow (next tank will have an insump skimmer). When I am home I run the skimmate level pretty high because I can check it periodically.
I have a three ring binder with the following that I leave for them:
1. A two page instruction sheet that covers daily maintenance and emergency information, ie AC/Heating companys' phone number, all our cell phone numbers & where we are staying if applicable, neighbor reefer phone number, where the generator is, how to start it, which pump to plug into it, etc.
2. A photo or printout of each fish and coral so that if there is a problem they will know what to tell me on the phone, not the funny yellow thingie is dying.
3. All my equipment manuals and instruction sheets are also in the binder.
4. There is also a complete table with a printout of my Aquacontroller settings, the AC codes and what each command line that I have programed is, but that is more for me so I can see/remember all my settings.
Good luck!
Joyce