Sandals in Cairo?

Are you a man or a woman? If man, no problems at all, not even in mosques, I think, if woman, I think (I'm not sure) putting socks on in mosques and very conservative parts of the country might be a good idea, but I'm not sure. I don't think this'll be a big issue.

The big fetish of arab men that sometimes strike western women as a bit odd is bare shoulders. This is the main thing to avoid. I don't think is similarily an issue, or at least, from my experience I never noticed this as a problem.

There's no problem wearing sandals in Egypt, but I wouldn't wear shorts anywhere in the country except at a beach resort. You will catch looks.

In mosques, you take your shoes off at the door, including sandals. No need to wear socks. If you're female, I would suggest carrying a small scarf you can throw on over your head when visting mosques. It's not necessary for non-Muslims, but it's a nice sign of respect.

Yep, agree with you both.
Egypt is however a pretty dirty place so your feet will be black at the end of the day.
If trudging through the desert, even around the pyramids the sand can get bloody hot on the old tootsies so maybe a pair of lightweight shoes.

Another point is that it's actually too cold now for sandals, at least in Lower Egpt.