I've been in Cairo in February 2017, I think for 8 or 9 days, just for fun. The Egyptian museum is one of the best options for sure. I've met a nice person there, Mustafa, at the waiting queue in front of the Cairo tower. He introduced me his mother and his son, told me that he works at the museum, and invited me to an exclusive tour through the museum. The pyramids of Giza is another 'best' option. It's easy to go there. There are so many taxis in Cairo you can simply find one that takes you to the entrance. Try to be there as one of the first person when they open so you can buy one of these limited permits to enter the Cheops pyramid. A third and option, but not everyone will like it, is to go to the center without any plans and explore the different quarters. It's no problem to get lost due to the omnipresent taxis that can bring you everywhere almost for free. Another 'must' is to visit two or three big mosques. You will be surprised how friendly the people in the mosques (even the imam) will welcome you. They are all interested to talk to you and show you the inside of the mosque. You must also see the Khan el Khalili quarter! It's not too far away from the museum so you could visit the museum and this quarter with it's bazaar and mosques. If you don't want to stay in this 20 Mio city with almost no tree and lots of dust and dirt, then take a tour to the pyramids south of Cairo. I've rented a personal guide and a personal driver, and they brought me in a full day trip to the pyramids of Saqqara and to Memphis, the ancient capital of of Egypt. You will see more pyramids, old Egyptian tombs and relicts than in Giza. Just let me know if you need more information.
Happiness is when what you think what you say and what you do are in harmony.