One of the coolest things in Cairo was going to the Nile at night. The banks of the Nile are apparently the Egyptian version of Lover's Lane. It was all teenagers and young adults who'd managed to get together and have a moment of intimate conversation (something I've heard is rare) or a clandestine holding of hands. There are lots of food and drink vendors and a general hang-out vibe permeates the place. The Cairo Tower reflecting off the river is also an incredibly beautiful sight. It's nice during the day, too! I'd recommend walking along the river at night, and spend the day admiring the river and its fisherman from the May 15 Bridge
You simply must take a Nile River Cruise while you're in Cairo - the panoramic views of the great city are really priceless, and it's the perfect chance to get away from the hustle of traffic in the streets and enjoy the beautiful sunset.