I think Denali is one of the best US national parks to visit in the summer months. Emily Hart After visiting all 63 major US national parks, I think some are best experienced in the summer. Although certain parks are more crowded in the summer months, many are also much more accessible. Without snow coverage, parks like Kenai Fjords, Glacier, and Grand Teton are easier to explore. I would argue that there is no bad time of year to visit the US national parks. But after visiting all 63 of them,