What actions can be taken as a free action?
Free actions are actions that are specified on the ship's action bar, other cards or special mission rules. You can normally only perform actions during the Perform Action step of the Activation phase.
Advanced Sensors allows you to perform any one of these free actions outside of the regular step at the cost of skipping that step.
From the X-Wing Rules reference (pg 12)

A free action is an action granted to a ship through a card ability or other effect. It does not count as the one standard action that a ship performs during the “Perform Action” step.
• A stressed ship cannot perform free actions.
• A ship can perform free actions even if it skipped its “Perform Action” step due to overlapping an asteroid obstacle or another game effect.
• If a ship is specifically granted a free barrel roll action or a free boost action and cannot complete the action in the desired direction, it may choose a different direction. It cannot choose to perform a different action.