A friend suggested "impromptu", and another "in situ".
Interesting.
The former is derived from a prepositional phrase ("in promptu"), and the latter still is one, unless it's written as "in-situ", but then is it an adjective or a prepositional phrase put into a modifier position with its orthography modified to conform to English norms?
Perhaps the question should be if there are any adjectives that don't have these endings and that weren't derived from other word classes.
Someone else suggested "yellow", a comment which I won't bother to respond to…