How can I safely average two unsigned ints in C++?

Your last approach seems promising. You can improve on that by manually considering the lowest bits of a and b:

unsigned int average = (a / 2) + (b / 2) + (a & b & 1);

This gives the correct results in case both a and b are odd.

If you know ahead of time which one is higher, a very efficient way is possible. Otherwise you're better off using one of the other strategies, instead of conditionally swapping to use this.

unsigned int average = low + ((high - low) / 2);

Here's a related article: http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html