If you want to hedge your bets, take a light-colored button-down cotton shirt to layer over a t-shirt. That's a combo I wore a lot. I could always take off the top layer if I felt I could be comfortable.
My favorite thing that I bought to wear in Egypt was a long skirt. I got mine at
SHUKR | Islamic Clothing for Muslim Women, Men and Children - it's basically a cotton mermaid skirt. I bought this one:
Embroidered Belt Skirt - but all the long skirts are beautiful and mine was very well-made and fit extremely well. It was as if they were designing for adult women with hips, imagine that. Anyway, I wore it all the time there, and felt totally comfortable. I was only sexually harassed ONCE in Egypt, which I understand is a pretty good track record.