I'm not sure I follow what you're saying when you say you "want some tiles to remain unchanged or unmodified by neighboring zones." As far as I can recall, a zone on one grid tile will not affect the neighboring empty terrain grid tile. You would have the land showing through.
As far as making a park on that land, there are plenty of Mayor Mode Plops (MMPs) that are out there, including dirt/concrete paths, trees, bushes, animals, etc. The LEX is a good place to start for that. Chrisadams' Rural Renewal Pack in particular has a fair number of options.
In other words, you don't need to cover over every inch of terrain on your map with a lot of some sort. I hope that answers your question.