I need to leave my cats alone for a month this summer. What do I need to get them?

I've left them alone for a week before and they were fine, if a bit messy. They rationed their many bowls of food and water that I left them. I mean, if I had to, I could leave a million bowls out for them for the month, but there's no way to gauge how many is enough. Any advice as to what to do to make sure they are okay on food and water, as well as their cat litter? I don't have anyone to come catsit for me, or even anyone to check up on them that I trust enough with my keys. I won't be able to check on them until the end of the month.