Crater Lake is very deep. 1943 feet deep, to be exact. But, there's much more to explore than just the lake. Book a stay at the Crater Lake Lodge (my recommendation for best views) or several miles away at the Cabins at Mazama Village. For camping in the summer, the Mazama Campground is situated in an old-growth forest. The Lost Creek Campground is a smaller, no-reservation area for tent campers only.
Besides the two Visitors Center (Steel and Rim), the Scenic Rim Drive is the best way to get acquainted with the park. The 33-mile route features 30 overlooks with scenic views. If you don't want to drive, a trolley is available. The narrated trolley tour lasts two hours and stops at several overlooks. Boat tours are also available if you want to get up close and personal with Wizard Island, a cinder cone island amid the piercing blue waters.
For hiking around the lake, the Watchman Lookout trail is a moderate 1.6-mile roundtrip that provides great views of Wizard Island. The Discovery Point Trail is a slightly longer but easier hike for lake views from a different angle. To see the popular Phantom Ship formation in the lake, the Sun Notch trail is a quick and easy half-mile roundtrip. If you're taking a boat trip, why not hike down to the water via the Cleetwood Cove Trail. It's a strenuous 2.2-mile roundtrip, but allows you to perch yourself on the rocks (or go swimming if you're unaffected by cold water).
For waterfalls, Vidae Falls requires no hiking whatsoever, as it is visible from the East Rim Drive. It's a narrow 100-foot waterfall. Plaikni Falls is a 2-mile easy roundtrip hike. These falls are shorter but wider. A nice walk through flowers and forest give a nice change of pace. Speaking of flowers, the very easy .4-mile roundtrip Castle Crest Wildflower Trail bursts with vibrant colors in the summer!
If you're looking for a longer, more strenuous hike, Mount Scott may be for you. This is the highest point within the park and offers a more complete view of the lake.