I'm assuming that you mean North America (but correct me if I'm wrong).
For landscapes, you'd essentially be looking at mainland USA or southern Canada. Mexico also offers some beautiful landscapes, but I'm not much of an expert regarding the country. You could go further up north to Alaska, Yukon, NW Territories, Nunavut, etc., but it's extremely cold if you go the wrong time of year, and can also be costly. Many places up north are unreachable by car, as they have no access roads.
In the US, you really can't get a bad shot in the mountain region. Yellowstone, the Grand Teton, or Yosemite come to mind. Montana is arguably one of the best places to get mountain shots in the US, particularly at Glacier National Park. Death Valley in Nevada is also quite a beautiful place, along with the Grand Canyon (no trip would be complete without it!). The Florida keys are quite far south, but worth the trip. The blue water amongst the chain of islands is gorgeous, and really separates itself from the rest of the country. Key West is definitely the place to go. Finally, the badlands in South Dakota and Wyoming are worth seeing, Devil's Tower is also quite a sight.
Canada has some great sights as well. Western Canada isn't extremely urbanized apart from Calgary, Edmonton, and Vancouver, so there's plenty of natural scenery, and national parks are abundant. Banff, Jasper, and Lake Louise are, IMHO, some of the most beautiful places to visit in the world, and they're definitely worth the trip. Perhaps the best place to photograph landscapes in Canada (again, in my opinion), is Waterton Lakes National Park. It's a little-known destination in SW Alberta, and the town has an overall population of about 80. However, the scenery is jaw dropping, the view reminds me of the test shots you see in Gordon's reviews from Queenstown NZ. You can't go wrong with ANY national parks in Alberta or BC. The prairies are also a great place to visit, if you've never seen land this flat, it's worth taking a look at. Lake Winnipeg in Manitoba is a great place on the prairies to get some beach shots, Grand Beach on the east side of the lake was ranked as one of the top beaches in North America, and truly is beautiful. The maritime provinces all offer excellent ocean views and that "east coast" look with pastel buildings and lighthouses. The Bay of Fundy is gorgeous, and needs to be seen in person to be appreciated. The east coast as a whole is beautiful, and all the people there are truly lovely folk.
Niagara Falls in both the US and Canada is a mediocre place to visit in all honesty, the falls have been so over-photographed that it's difficult to get an original shot, and the area has been so commercialized that it's lost all of its charm. Niagara on the Lake in Ontario is nice, but there's nothing overly exciting to photograph there, aside from some of the vineyards.
Hope I helped!
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"There are no rules for good photographs, there are only good photographs."
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