1) Hire a car, if not for the duration, for at least one day, explore the island, you won't regret it, aussuming you can drive. If you don't drive, get out and about on the buses to see more of the island.
2) Ibiza town, afternoon and evening is amazing. Start from the very top at night and walk down from the castle into the crazyness of the town at night.
3) KM5.