I believe you could easily minor in math or physics. Computer engineering usually requires you to take a lot of math already. And as for physics, you will usually take lower divsion physics. The nice thing about computer engineering is that it combines computer science, electrical engineering, math, and physics together nicely.
If your computer engineering studies emphasize networks, I recommend the EE minor. But this is only from observing friends who are IT guys and electrical engineers. I'm neither.