If you are coming out West, you'll probably be hitting the "Grand Circle" of National Parks in Southern Utah. This should be your first stop since you are leaving in the Spring. Waiting until the summer will have you in the hottest times of the year there. Yellowstone and the Grand Teton can wait until summer as they are really at their best, temperature-wise, at the hottest times of the year, and that also includes Glacier and Olympic National Parks. Another great stop is Mt. St. Helens National Monument in Washington. If you have your passports, you can visit Vancouver Island, catching the Black Ball Ferry in Port Angeles, Washington. Just a couple of suggestions.