One of his most untypical roles (I guess) is in something very famous. In the "commune" part of EASY RIDER, he plays a sort of guru figure making a very small speech.
One of the first things I think of (even though he has the closest thing to a "thankless" role in it) is a very underrated 1967 comedy called THE HAPPENING.
And he got to play the lead role in the comedy ENSIGN PULVER. It's considered a very weak sequel to MISTER ROBERTS, but I've always been fond of it.