The Siwa Oasis is the source for a significant portion of Egypt's freshwater supply. It is located in the sparsely populated, far west region of Egypt, only 30 miles from the Libyan border. Because the Siwan people are so isolated from the rest of the country, they have developed their own language (Berber), culture, and customs (women, although rarely seen in public, completely cover their bodies, including the face).
As a tourist, there is much to do in Siwa, including desert safaris (which include sand surfing!), donkey-driven village tours led by local children, and exploration of wondrous oracle ruins. There are also plenty of delicious dates and olives to eat, provided by the thick palm groves and olive orchards that surround the city.
Getting to Siwa is a bit of a journey from Cairo (5+ hour train, and then a 4 hour bus ride), but it is definitely worth the trek, especially if you're looking to escape to some peace and quiet after the extreme crowds and liveliness of Cairo. And if you thought things in Cairo are cheap, prepare yourself for the amazingly affordable prices in Siwa!