In the sence of the words that I believe you are trying to use:
A simple sandwitch to me sounds like a sandwitch that isn't complex. Perhaps this sandwitch only has one ingredient?
A plain sandwitch doesn't work as well. To me it sounds like a sandwitch with nothing in, which is of course not a sandwitch at all!
In this case, a simple sandwitch is really the only practical option. But even then, it doesn't read that well, as we don't often describe sandwitches as simple.
A plain place to me suggests a place that is bland, without character or decoration. For example, a whitewashed village, with no decorations.
A simple place doesn't work very well in most cases. It sounds awkward, and doesn't convey the message that the place is plain or bland. Instead it would suggest that maybe the people there have simple needs and simple desires.
Student or Learner