"I want to eat healthy things."
The link has somewhat a simple classification but when it says "optional" it actually mean "both ways exist".
In this particular sentence I think we better use 的, as there is no close association between Health and Thing in meaning.
If we change the noun to "food" 食品 then we can see better the difference. 健康的食品 would be close to "healthy food", whilst 健康食品 would sound like "health food".