Find the median of 3 (4,5?) numbers
Matrix or Vector products.
Any of the sorting algorithms, or maybe just the merge operation (I feel like it might be interesting but short in mill asm).
Towers of Hanoi (recursion)
Basically almost everything on Rosetta Code will be interesting, and have sample code in C and assembly. Of course some of these may be too complicated or require os-specific console input/output.