Nice upkeep of foliage, trees, tables, trash cans, and paint all help guests feel so much better in a park. What's sad here is that — just as Matt Ouimet said — not enough industry professionals seem to understand that fact. Some things can't be immediately helped, but change can happen over the course of a few years.
Something Kings Island really needs is a renewed interest in trees and foliage. Little displays like the floral calendar, carousel horse, and season-marking Eiffel Tower display (behind the Grand Carousel sign) are really nice. On the other hand the Floral Clock has been broken for a while, the Eiffel Tower topiary often looks like a... well a... poorly kept topiary. The park could also place some non-foliage-based displays if they don't want to put strain on the gardening teams. If I had to choose three areas of Kings Island to plant more trees, it would be
Camp Snoopy, specifically near Franklin's Flyers
Rivertown, specifically near Diamondback's entrance and near the path at the area formerly known as Swan Lake
Vortex's plot (this one is very specifically foliage that a new coaster would go around and/or that would compliment a new coaster)
Action Zone is somewhere on this list, but I would tie any new trees and foliage there directly into a larger area redevelopment rather than somewhere as a quick fix. I'm sure there's a worry about obstructing some sightlines and getting trees too close to coaster tracks as well.