I keep some bags of frozen steamable veggies in the freezer. When I have one of those "Oh no, I forgot to make veggies" moments, I throw one in the microwave and it provides a day's worth of veggies.

Pickles are also a great "oh no, I forgot" option, though the salt will make you retain some water for a day or two.

Going to have to try one of those celery root chip ideas. I wonder if they'd crisp nicely under the broiler...

Cooked spinach
 

I take half a bag of baby spinach and throw it in a frying pan with a little olive oil (very small amount), garlic, salt and pepper. It cooks down to a tiny portion of the original size and it's delish.