If you like Bartok, I think the Sonatina Sz 55 will be a good choice. I've personally taught this work to students that are either really young with small hands and short attention spans, or students that are really busy with their lives.
The Danzaz Argentinas by Ginastera is more challenging and would also add some contrast to your program (in terms of ethnic styles -- the more latin feel would be a nice contrast from the Oiseau tristes -- though the motoric/perpetual motion may not neccessarily be that contrasting with your mendelssohn etude or the fantasie impromtu).
The Scherzo by Oscar Morawetz. This is a Canadian piece that sounds a harder than it is to play as it repeats itself. Examiners tend to like this work as it is not that commonly played.
Alexina Louie's Memories of an Ancient Garden is also really effective in the exam, as in involves playing, hitting and strumming the strings and structures inside the piano. (they have removed this from the syllabus, but it can still be done as an own choice).
My personal favourite is the "In Memoriam to the Victims of Chornobyl" by Larysa Kuzmenko, a composer from Toronto. I personally played this piece for my ARCT exam, as well as by LLCM (london college of music) exam ... I received perfect scores in both exams for this work. It is easy to learn (in my opinion) as the composer has included detailed instructions for what each section of the work means to her. It has also the most variety of ethnic quotes, textures, technical and musical elements of the works that I have suggested.
Best of luck with your exam!!