Niagara Falls is one of the world's great natural spectacles: 2,600 feet of falling water, 600,000+ gallons per second, and mist you feel from a block away. The Canadian side has the panoramic view of Horseshoe Falls; the American side gets you closer to the water at Goat Island and Cave of the Winds.
The falls are the headline, but the region rewards a longer stay: Niagara-on-the-Lake is one of the prettiest small towns in North America, the surrounding peninsula is serious wine country (the world capital of icewine), and the gorge below the falls offers jet boats, a whirlpool, and scenic drives.
Crossing the border is easy with a passport (walk across the Rainbow Bridge for the best free view), and the two sides complement each other. Most visitors should see both.
Niagara suits couples, families, and anyone pairing it with Toronto (90 minutes), Buffalo (30 minutes), or a Great Lakes road trip. Two to three nights is the sweet spot.