This isn't really a playmaker question, this is a game mechanics question.
In most game situations, you can handle almost everything with Triggers and Events. When a player steps on an invisible cube on the path, a series of actions are called – like a boobytrap in Indiana Jones…. So in your game the player crosses a trigger and the sprite actions are called with an event.
Playmaker uses function state machines which makes game interactions like this easy to build. You have an invisible trigger on the path, with a FSM that is waiting for a collision with the player. When player crosses the trigger the FSM calls an Event that sends its active "state" to the next FSM where "stuff happens" – enemies are activated, score variable increased, particles explode, etc…. The FSM can even deactivate the game object it's attached to, so once triggered it it can spawn other objects, send events to other FSMs, then destroy itself. It might reset itself, or it might only work the one time, that is up to you how you build it.