'parsimonious' is a pretty decent word here
we could try defining a new word for specifically this sense of 'simple'. for example, 'fivous', 'blorsy', 'bonatchy', 'jatomurous', or 'wislous'. the problem is that if the word gets adopted, it's likely to undergo the same sense shifts as its synonym 'simple'
But there are some things that are just simple, like a Bankers Box.