If you mean the "simple physics" feature in stencyl; maybe a Sokoban game (box pushing puzzle game).
If you mean a physics-mechanics-based, simple game; try making an
Asteroid clone. You can get some fun movement going with just basic physics blocks and collisions.
Another classic Atari game (even simpler than Asteroids) is
Lunar Lander. Always a fun time; it could be done easily with the most basic of physics blocks.