What is a good substitute for arugula?

This is actually kind of an oddball version of soupe au pistou -- most versions don't include an additional green leaf beyond basil or any nuts in the pistou, consisting entirely of basil, garlic, olive oil, cheese, salt, and often a little tomato. If you don't want to use the spinach, you could just follow a more traditional recipe. Here's an example from a good source:
https://www.davidlebovitz.com/soupe-au-pistou-french-vegetable-soup-recipe/

I don't see arugula in the recipe, I see spinach. Any delicate, richly-green leaf should do. Perhaps something like baby kale or watercress?