A legend tells that when the first visitors got to Crater Lake they took pictures and sent them to Kodak to print. Kodak, very apologetically, offered to reimburse the cost of printing, given that the blue of the lake was so unnatural that it could only have been an error in the printing process. And it is exactly that unique color that brings every year millions of tourists to southern Oregon to visit Crater Lake.

Some basic info on the park

Crater Lake is a dormant volcano and is the only national park in Oregon. It is located in the southern part of the Cascade Range, the mountain range that spans from northern Washington State to northern California and includes other gems such as North Cascades NP and Mt Rainier NP in Washington and Lassen Volcanic NP in California.
If Mt Rainier is famous for its flowers, Arches NP for its arches and Yosemite for its waterfalls, Crater Lake is known worldwide for its intense blue. This color is the result of more than 1,900 ft of depth and is the main reason why you should not miss this national park.
The park is covered with snow for much of the year, and in some years it is possible that the access roads open only in summer.
Although Crater Lake is so famous, you will not find the usual dozens of gateway towns around the park. Unless you booked months or years ahead almost certainly you won't find a place in one of two lodges in the Park (Crater Lake Lodge and Mazama Village Motor Inn). Outside of the park there is not much choice. I would recommend the Prospect Historical Hotel in Prospect, with excellent rooms for a little over $100. You can choose either a motel room, with TV and breakfast included, or try one of the bed and breakfast rooms (breakfast included), more characteristic but without tv.
Crater Lake's location is not ideal if, like many other tourists, your trip is a one-way from Washington State to California. In that case, unless you want to retrace your steps (and lose a lot of time) you must decide whether to visit Crater Lake and miss part of the Oregon Coast (the part of Bandon, one of the most beautiful stretches on the Pacific Coast, in my opinion) or skip the national park and stay along the coast (same type of problem for Lassen Volcanic NP, just south of the Oregon-California border). Giving you a personal advice is as hard for me as choosing between a cheesecake and Boston Cream Pie. It really comes down to your personal preference. What I can tell you though is that ideally, instead of doing a one-way,  you would want to consider a loop from Seattle down to northern California along the coast and then coming back north following the Cascade Range. This is the kind of trip I did in 2007.

What to do

Crater Lake is a rather small park, and you can get a lot out of it in one day or even a few hours. Although there are several opportunities for hiking, the real attraction of Crater Lake is the lake itself. The scenic road that runs around the lake leads to dozens of viewpoints, each one of which will give you a slightly different view of the lake. While all are spectacular and offer different perspectives, I must honestly say that after a while they are also kind of repetitive. If you're the kind of person who prefers to visit a lot of places without spending too much time in each, Crater Lake will be a great stop, and even with three hours you'll enjoy a lot of what the park has to offer. On the other hand if you like parks with more opportunities to get lost in the wilderness or long hikes then you you'll find it hard to come up with enough attractions to fill up a whole day. Not that Crater Lake hasn't much to offer, but it is difficult to find a lot of high level attractions like you would in parks like Yosemite, Glacier or Zion (where you could easily spend a week in the park and see or do every day something completely different and thoroughly amazing).
Having said that, I would recommend at least half a day to visit Crater Lake, maybe a whole day to take it easy, do a couple of hikes and take the boat to Wizard Island. This is an cone shaped island that can be reached via boat and then explored on foot. Please remember that to get to the boat you'll need to hike the very steep Cleetwood Cove Trail down to the lake shores, about 700 ft below street level. Going down is not difficult, but remember that you must also hike back up on the way back. The trip takes less than two hours unless you stop at Wizard Island, and potentially much longer if you decide to explore the island. Remember that once you get off the boat and on the island, access to the boat is allowed only if there is space. If there isn't, you'll have to wait patiently for your turn.
While you drive on the Rim Drive, there are a few attractions you should not miss. The Pinnacles is an area with strange rock formations that remind a little bit of Bryce Canyon's Hoodoos.
Another very interesting viewpoint is the one that overlooks the Phantom Ship. The ship is nothing but a basalt rock formation but it has a great charisma and reminds of scenes seen in a Pirate movie.
Another must stop is at Vida Falls, beautiful waterfalls pretty much along the road.

Ultimately Crater Lake is a beautiful park to visit and a great option if you are looking for something that can give you a lot in a relatively short time. But this is also, in my opinion, the limit of Crater Lake and the reason why this park is not among my personal top-10 National Parks. If you want a park to be explored through long walks you have better options in the area: Lassen Volcanic NP or, further north in the state of Washington, Mt Rainier NP and Olympic NP. However Crater Lake is a not to be missed jewel of the Cascades and you should plan at least a quick visit.

Rogue-Umpqua Scenic Byway

Not far from Crater Lake, you can experience a drive along one of the most famous American Scenic Byways. The Rogue-Umpqua Scenic Byway technically starts from Gold Hill and ends in Roseburg, just under 180 miles that will take you through the Umpqua National Forest, with dozens of waterfalls and beautiful views along the road. The really interesting part (in my opinion) of the Rogue-Umpqua Scenic Byway is the stretch between Roseburg and Diamond Lake, where you will find all the waterfalls and a couple of other great attractions. I'd recommend you stop at a ranger station to get one of the maps  with all the major attractions along the way. There are dozens of waterfalls, some literally on the edge of the road, other will require a short walk or a longer hike. After you leave Diamond Lake the first waterfall you shouldn't miss is Watson Falls, 272 ft tall and absolutely magnificent. Just keep in mind that this waterfall may be almost dry during the hottest Summer months.
A second great stop is Toketee Falls, whose shape is as unique as it is pretty. This one can be accessed only via a short walk. Continuing west another couple of stops worth considering are Fall Creek Falls and Susan Creek Falls.
Shortly before arriving to Roseburg, do not miss one of the most unique attractions of the Rogue-Umpqua Scenic Byway: The Colliding Rivers. This is a point where the North Umpqua River and Little River clash face to face, causing, in the wettest months, a spectacular show.
The Rogue-Umpqua Scenic Byway is a great day trip from Crater Lake, and if you want to reach the coast from Crater Lake (or vice versa) there is no doubt that this should be the way to go.