When you are traveling south from downtown Cairo along the Nile you might notice a peculiar looking building sitting near the southern tip of Rhoda Island. That building is the top of the Nileometer.
There is a description at this Wikipedia link and a longer history at this serious water scholar's site. Suffice it to say that with the rise and fall of the Nile each year playing a central role in Egyptian life, the government kept a close watch on it. Of the many Nileometers that once existed, this one is the best preserved.
For a 15 pound entrance fee (about $2 U.S) you get a nice tour and possibly an explanation from a guide. (tip extra, of course.)
It is an interesting device. Lit with a mixture of both incandescent and fluorescent bulbs, it will just about drive you nuts trying to get a good picture.
A couple of blueprint drawings are on display along with some narrative, but you will need to get most of the details elsewhere.
(pre-Photoshop version on the right)
Also in the immediate area of the Nileometer is the Om-Kolthoum Museum and the Al Manesterly Palace. Om-Kolthoum was an exceptionally well known Egyptian singer and actress. Known throughout the Arab-speaking world, she was and still is extremely popular.