If they have vehicles or investments, make sure all of their registrations, titles, and investment certificates are safe and easily accessible. Vehicles are useless without a title and I spent a year tracking one down because the original wasn't found. DMV has no right to provide you one until the car is in your name, but you can't put it in your name without the title. Couldn't ask the owner, for obvious reasons.

If they have shares, you need the original certificates because it costs hundreds just to make new ones and you can't sell (or transfer) their stocks without them. This part alone added months to probate for me and its the last step.

Learned all this the hard way.