Definitely include SPI/I2C, timers, and timer capture; these come up a lot when interfacing hardware. You may or may not want to go into the structure of GCC, the AVR-GCC lib, and the Arduino macros; I've had to dig into them for advanced projects. (some commands interact, especially commands that use timer resources.) For the C basics, iteration, selection, types, and functions are obvious topics. If you have time, common syntax and logic pitfalls, and writing readable code (and documentation) are issues.