Is it a sensation? I think that's the best fit. It is an observation, but it's an observation of internal reality rather than external. That seems acceptable to me - the human mind is part of nature too - but it might seem like a slippery slope. And it is very different from the traditional senses.
I'd vote for this one. In Zen the mind is sometimes referred to as a thought generating organ. With this in mind (pardon the pun), observing one's thoughts might be similar to observing one's breathing. Or observing any other bodily function.