I went to Alexandria alone for a long weekend and I hate to say it, but... I wasn't that impressed! The library was super expensive (unlike other famous Egyptian sights) to visit, the hotels and restaurants were quite dowdy and I was hassled a lot by men, women and children! Perhaps I shouldn't have gone alone? Perhaps I went at the wrong time of year? I'm not entirely sure.
Nonetheless, it is worth a visit to see the fortress and the underground catacombs. There is a lot of history in this place and the refreshing Mediterranean Sea breeze as you walk along the corniche is a welcome change from Cairo's haze of pollution. Perhaps call in for a short trip and that will be just enough of an experience.