Basically anything you can put in there to generate a supply of ammonia. Spinach sounds like a plan, but fish food would work too.
Since you're possibly wiping out all the bacteria in your tank, here's an idea: your bacterial colony will grow to match its food supply. So, if you provide a lot of food (ammonia) to an already established colony, you would cause it to proliferate. Then, when you're done treating your tank, you can take your now much larger colony out of isolation, drop it into your tank and hopefully have more bacteria than you actually need. Basically, overfeed your isolation jar (not like you have any livestock in there to worry about) and with luck your colony will get beefy enough that you don't have a mini-cycle when you reintroduce it to the main tank. Note: take this with a grain of salt, I'm basing it on an understanding of how bacteria works rather than any personal experience.