July: Rim Drive usually opens fully in July, and the Cleetwood Cove trail and boat tours give you the only legal access to the lake's surface. The water is at its deepest blue in clear summer light.
September: The rim is still open, the crowds are gone, and September's clear air shows the lake at its deepest blue.
Permits and reservations
No entry reservation or timed entry.
Backcountry permit: A permit is required for overnight backpacking. Pick it up at the ranger station at park headquarters.
Boat & trolley tours: Boat tours on the lake and trolley tours around the rim are booked through the park's concessioners (seasonal, summer).