The Citadel is worth at least one visit. It is very hard work, especially during the 12-3pm period, with many steps and seemingly unendless cobbles, but there are some restaurants and shops to ease you and an art gallery halfway up. It is hard to recommend the citadel for its contents though the 2nd Bn. Parachutistes of the Foreign Legion put on a show every year, discussed below, which is worth going to, but the views are simply amazing, especially at sunset.
Come during the 15 Août, and you will be treated with the best fireworks show you have ever seen. Go to the main beach and you will see the show proper; each year has its own theme. We went in 2006 and saw the 'Apocalypse' themed show, which included fireworks evoking fire, brimstone we were hit on the head numerous times by falling caps--not a great idea to sit on the street directly beneath it and volcanic eruptions accompanied by loud, germanic classical music and an accompanying video projected onto its largest wall. In the daytime, the Parachutistes put on a show and do a jump from 15,000ft, landing in the sea. Best viewed from the bar U Cornu Marinu, if only under the pretense of avoiding flying boots.
L'Eglise Santa Maria
is worth a visit; you can't miss it, it's the 'Big Pink Church'. It is a lovely old-style Catholic church evocative of the Orthodox style.