The DTE RS-232 module is a tight and tidy way to convert RS-232 (computer-level) signals down to a microcontroller level signal strengths. DCE means it's configured as a "Data Communications Equipment" (with 9 holes).
The Solarbotics Arduino Freeduino Enclosure (SAFE) is a laser-cut acrylic case for your Arduino Mega. It also has the added feature that it can house an Arduino or Freeduino instead with a MEGA amount of space left inside.
Throw this bundle together to have one of the first Arduino-compatible Mini-Sumo kits out there. Get your trusty soldering iron out, build the Sumovore and Sumoshield Kits, then toss the Arduino on-top, program it with the supplied code, and watch the Sumo/Line-following madness ensue.