Pack light, breathable clothing, one modest outfit covering shoulders and knees for temple visits, comfortable closed shoes, and a light layer for evenings — the ship supplies towels and toiletries, and usually offers laundry.
What the temples themselves require
Dress modestly, especially at religious and historic sites, is standard travel-advisory guidance for Egypt generally, not just a suggestion — shoulders and knees covered is the practical baseline to pack around.
Footwear matters more than anything else on this list
Karnak, the Valley of the Kings, and Philae all involve real walking on uneven stone and sand in open sun — comfortable closed shoes beat sandals for a full excursion day, whatever the temperature.
What you don't need to bring
Photography with a mobile phone is free at government-run sites, so there's no need to plan around paid photo permits for casual phone shots (a separate camera fee can still apply for larger cameras — see What Should Photographers Know Before an Egypt Trip?). Laundry service means a week-long cruise doesn't need a week's worth of clothing packed.