When in Egypt, do as Egyptians do. Although this doesn't mean that you'll need to don a veil, you'll feel more comfortable during your visit if you take your cue from the people you see around you. Egyptians, both men and women, tend to dress modestly. Unless you're at the beach, leave your shorts in your suitcase. In summer, opt instead for light cotton pants and skirts, preferably knee-length or longer. Short sleeves are fine for both men and women, as are T-shirts and jeans. Dress more conservatively if you're planning on visiting any mosques or churches; women should bring along a light scarf to throw over their heads if necessary.
You'll see Egyptian couples walking down the street arm in arm or holding hands. Less discreet displays of affection are frowned upon. Good friends, both men and women, will greet one another with a light kiss on both cheeks. When in doubt, a handshake is fine, although more-traditional men and women may not shake hands with members of the opposite sex.
Egyptians place a high value on politeness—to be called impolite is considered a true insult and implies that one was not raised well. Knowing only how to say please (min fadlak to a man and min fadlik when addressing women) and thank you (shukran) can make your interactions much more pleasant.
If you're invited to someone's home, a small gift is in order. Flowers and pastry or some sort of sweet are always appreciated. Don't bring alcohol unless you're sure that your hosts drink. In any situation it is considered rude to put one's feet on the furniture or table. The more traditional taboo against using one's left hand to eat has become mostly a moot point these days, as communal meals are less common and eating utensils are the norm. Still, if you find yourself sharing a meal from the same plate with someone, use your right hand to take food from the main plate.