**Yes, your itinerary is generally possible—but your Canadian Working Holiday visa does not authorize entry into the United States.** Your U.S. entry requirements depend on your Belgian citizenship.

Belgium participates in the **U.S. Visa Waiver Program**, which allows eligible citizens to visit for tourism for up to 90 days without a U.S. visa. For your 3–4 days in NYC, you would normally need:

- A valid Belgian **biometric passport**.
- An **approved ESTA**, applied for through the [official government website](https://esta.cbp.dhs.gov/). Apply when you make your travel plans, at least 72 hours before entering the U.S.
- Your NYC–Brussels ticket and details of your visit available for border inspection.

This assumes you meet the program’s eligibility requirements. Certain travel history, dual nationalities or other circumstances can make you ineligible; in that case, or if ESTA is denied, the alternative is to apply for a **U.S. visitor visa (B-2)** before travelling. ESTA approval allows you to request entry; the border officer makes the admission decision. [U.S. State Department requirements](https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visa-waiver-program.html)

There is also a practical detail about Niagara Falls: **Niagara Falls International Airport (IAG) is on the U.S. side and currently lists no nonstop NYC service.** Nearby **Buffalo Niagara Airport (BUF)** does list flights to NYC-area airports. You could visit the Canadian side of the falls, cross the border by ground transport, then fly from Buffalo. Your U.S. immigration inspection would happen at the land border, and **ESTA is required for that crossing too**. [Niagara Falls destinations](https://www.niagarafallsairport.com/traveler-info/where-we-fly), [Buffalo destinations](https://buffaloairport.com/flight-info/air-carriers/commercial), [CBP land-entry rules](https://www.help.cbp.gov/s/article/Article-1291?language=en_US)

You can then spend your few days in NYC and fly directly to Brussels; you do not need to return to Canada first.