Mostly ninja'd but hey. I like raw spinach leaves as a calorie-free snack food. Kale is good too. Never liked iceberg lettuce, raw onions, or creamy salad dressings... But there's something simple and comforting about munching raw leaves.
Cats actually eat a little grass to help their digestion! Dunno if spinach counts for that though. It was funny to watch her viciously nom at the blades until her pointy predator teeth finally cut them. I think most "indoor" cat foods have supplements for that.
Couple of late notes, but:
1) We don't actually know why cats like to eat grass.
As best as we can tell it doesn't have any real digestive benefits, nor do they really get anything out of it. The only effect it seems to have is to encourage vomiting. (Doesn't stop companies from trying to sell you cat grass though).
2) Note about kale, but don't eat raw kale. Kale, (along with a handful of other plants like brussel sprouts) is a goitrogenic vegetable. That means that when eaten raw it has a handful of bad effects, like inhibiting iodine processing in your thyroid glands, and playing absolute hell with your liver and kidneys resulting in kidney stones. The solution to this is rather simple, before eating kale boil it for about seven minutes, drain, and squeeze any excess water out. That reduces the amount of goitrogens by about 90%, while still keeping the vast majority of the healthy vitamins and minerals that it contains!