Explanation:

- "Excursion" refers to a short journey or trip for leisure or exploration. It often implies a brief or short-term travel experience.

- Option 1: "Holiday" - Generally refers to a period of time when one takes a break from work or routine, often involving travel or relaxation. It is broader in concept than an excursion.

- Option 2: "Tour" - A journey or trip from one place to another, often for pleasure or exploration, similar to "excursion." It typically includes visits to several places and can be short or long.

- Option 3: "Boating" - Involves activities on a boat, often recreational. It's a specific type of excursion involving water travel.

- Option 4: "Trekking" - Refers to long, challenging journeys, usually on foot and across difficult terrain. It's more physically demanding than a typical excursion.

The synonym for "excursion" is Option 2: tour.