Sandals are fine, as are the capris. I'd make sure the t-shirts were big and flowy and not tight-fitting. 3/4 sleeves are a good idea, but sleeves that stop just short of your elbows are OK, too.

I'd recommend a large cotton scarf, actually, rather than silk, because in addition to quickly making you mosque-friendly, you can use it to mop your forehead when you get hot. It also makes a nice swimsuit wrap if you're staying at a Western hotel w/ a swimming pool (or taking a Nile cruise w/ a pool.) On my most recent trip to Cairo, I used a pareo I bought in Mexico and it was perfect for all of these things.

And speaking of the heat, a wide-brimmed hat is a definite must.

I traveled all across Egypt w/ my best friend when I was in my early 20s and had a grand time, you will too. Definitely don't be afraid to strike up a conversation with Egyptian women -- many speak English or French quite well, particularly in the cities.