Niagara Falls is a sight to see, but located south of Seneca Lake in the Finger Lakes region lies a lesser-known fantasy-like area called Rainbow Bridge and Falls in Watkins Glen State Park, New York. Maybe on Montreal trip?
Ice cave in the Skaftafell National Park in Iceland. Ice caves collapse and new ones form as each glacier grinds slowly towards the sea. Local guides can help you (without dying) find one. (Visit Skaftafell National Park in Iceland.