For some reason the SFBT High Wealth Parks don't have a path-less filler piece like the other sets do (excluding the diagonal fillers but those are for placing next to diagonal networks) which means every single tile has to either contain a path or be mismatched. Does such a piece even exist? Where can I find it?
It was ommited because it would be equal to the empty grass tile included with the game itself. Now, I'm not sure if some mods that change grass textures override that equivalence, but is possible.
I guess matias93 is right, the in-game 1x1 grass lot can be used for that purpose, hence ArkenbergeJoe probably didn't include one in his set. Feel free to create your own lot(s), maybe some 2x2 grass lot with trees for creating larger parks, if you like.