Most of these would be more difficult than Ligeti's Arc-en-ciel (etude no.5) or some of the pieces from the Musica ricercata anyway!
Here is a list of some possible things (from Stephen Drury's Contemporary Piano Summer course repertoire list):
http://www.stephendrury.com/SICPP/repertoire.htm
If you really don't like Ligeti, I think Takemitsu's Rain Tree Sketch (either I or II) or John Adams's China Gates would do quite nicely.
Eugene