take a walking tour and don't forget to check out the ruin bars, and make sure you go to a spa, and also the terror museum is worth a look at for some sobering history.
Insider Tip:you must see the musical fountain on margaret island. the music plays on the hour from 11am and plays both classical and modern pieces, with the fountain playing in time with the music. in the evening there is also a light show along with it. it is truly spectacular and noone should miss it! google beforehand for the times and music, as i think it only runs during the summer.