From almost any place along the Corniche, you can see a short, squat castle perched on the end of the breakwater at the outer rim of the eastern harbor -- this is Qaitbey Fort. For fans of military history and kids who like forts, Qaitbey is a great place. Austere and solid inside, you can wander the little rooms and peer across the harbor toward the town through the narrow windows of the upper floors, or check out the massively thick-walled sea side of the fort where they have a few old cannons. There's a small, high-roofed mosque in the center of the fort that you shouldn't miss, more for its simplicity and austerity than any elaborate tile work or architectural flourishes. Unfortunately, Qaitbey lacks any documentation or exhibition of artifacts.
The fort may seem to be in remarkably good condition given its age -- it was built between 1477 and 1480 during the reign of the Sultan Qaitbey -- but this is due to extensive restoration to repair damage from, among other sources, a British bombardment in 1882.
If you came to Alexandria hoping to see the famous lighthouse, this is as close as you're going to get: The fort was built on the site of its foundations about 175 years after it collapsed in an earthquake. Though the lighthouse stood on an island at the entrance to the harbor, the fort lies on a causeway, which was built to provide defense against potential Turkish attacks, that's accessible by foot. Rubble from the old lighthouse was likely used in the construction of the fort, and there's a theory that the large, red, granite pillars incorporated into the outer defenses came from just this source.
The small gift store lacks any souvenirs specific to the fort, and the toilets are all right.