Don't go in the middle of the Summer.
The last time I saw so many people in one place, I was in line at Disney World. The sweltering heat and shoulder-to-shoulder crowds made it difficult to take in the majesty of the town. Between mid-April and early June are the sweet spots for enjoying a swim and a hike. Crowds will be at their worst from June through August. Mid-September to early October is less crowded, but not all trails may be open and the rainy season begins. Staying in a town overnight is a great way to enjoy the town after the day-trippers go home. However, "high season" runs April through October, so book a room at least three months in advance.