I would imagine if people transition gradually, it might not be so hard.

Pumpkin seeds; put in salads, soups, yogurt desserts, smoothies, sandwiches

Spinach: Add to sweet fruit smoothie if you don't like spinach. Or put a little in salads, soups, etc.

Buckwheat: If you don't like it (I LOVE it mixed with Humus and frozen green peas, but I know, not everyone has the same taste), mix it with rice, or quinoa, or millet, or any other whole grain - even steel cut oats

Black beans; Doesn't everyone love Mexican food? Mix with chipotle sauce, salsa, add chicken, etc and put it in a tortilla, wrap, taco, etc.

The great thing about the suggestions above is you can actually create not just one meal but lunch and dinner in about 5 minutes (not including the cooking time; but the prep time can be as little as 5 minutes)