Oh, I do apologise, RSD; I have just seen this. I am going this Friday.
Right OK - well I don't think that heat is going to be a big issue then. Be warned that it may be colder than you had expected - believe it or not, snow in Egypt at this time of year is not unheard of. You could also see some rain and a fair bit of wind in the general Cairo area and the north coast. Will likely be very windy down the Red Sea.
Grand Egyptian Museum is now (mostly) open - 11 years later than planned but project timelines in Egypt are something that you just learn to laugh at once you have lived there for a while. I haven't been to the GEM yet - it only opened late last year, but from all accounts it is massive and you could probably spend the best part of a day there.
Pyramids/sphinx etc will be a one day tour.
The other must do in Cairo (in my opinion) is the
Khan el-Khalili - Wikipedia - late afternoon and evenings is the best time. On the other side of the street is the
Al-Azhar Mosque - Wikipedia - well worth a visit. If you have time then also in the same area is
El Moez Street in Old Cairo: A Detailed Local’s Guide
I wouldn't go to Ain Sukhna - it is more a place for Egyptians to go to escape Cairo for a few days and relax - its not really a place that Western tourists go to. Gulf of Suez is mostly sand and water - there actually isn't a lot more to see than that.
So I would recommend heading to Luxor instead - fly down and back - spend a couple of days there - Valley of the Kings, Luxor Temple, Karnak etc etc.
As far as safety etc - frankly I feel safer walking around Cairo of an evening than walking around Melbourne.
Food wise - avoid shellfish, lobster, prawns etc - apart from that I just tuck in - as always - look for places that are popular with locals. The Patisseries in Cairo are well worth a visit too.