It's actually as easy as:
DivElement elt = Document.get().createDivElement();
elt.setInnerHTML( SimpleHtmlSanitizer.sanitizeHtml(myHtmlString) );
String plainTextString = elt.getInnerText();
Or you could of course use code similar to the SimpleHtmlSanitizer that strings *all* the HTML tags, instead of just the non-white-listed ones; that would save you from the setInnerHTML/getInnerText dance so could possibly perform better (and the "sanitizer" code would probably be simpler as well)