-these kinds of things
-this kind of thing
But do we say: What kinds of things or what kind of things?
It depends on whether you want to talk about one kind of things or many kinds of things.
What kind of things did you see? <-- The speaker assumes only one kind of things were seen and asks what kind that was.
What kinds of things did you see? <-- The speaker assumes more than one kind of things could have been seen and asks what kinds those were.