Crater Lake National Park is a beautiful place to visit and I would recommend it to anyone who wants to visit Oregon. There's plenty of camping in the area and you can also stay at the lodge for more comfortable accommodation.
The park stays open year-round, though they average 40 feet of snow in winter so keep that in mind. In winter there is cross country skiing, snowshoeing, and other winter sports available. Check for road closures, as the only entrance open during winter is typically the Southern Entrance off of the Crater Lake Highway (closest to Medford, OR). The lodge is only open during the summer, so keep that in mind.
We chose to stay at the Mazama Campground which is about a 15 minute drive from Crater Lake. If you want to stay at the lodge, it does book up in a couple months in advance so keep that in mind. There are several campgrounds about 30 minutes from Crater Lake if the ones close by are full. Lost Creek Campground is a nice one and is also cheaper than Mazama.
There is a small shop at the Mazama Campground and a gas station, though prices here are expensive and the food is relatively limited (granola bars, mac n cheese, ramen, etc.). Next to the gift shop up at the rim there is a cafe with sandwiches and coffee. The lodge also has a very nice restaurant and a coffee stand that is open in the mornings.
If you only have time for one hike, do Garfield Peak. Check for closures though, as the hikes close during winter when there is snow as it is very slippery with snowfall (at least one person a year attempts to hike a closed trail and has to be rescued by the rangers). Garfield Peak gives a gorgeous view of Crater Lake and only takes a couple hours (3.4 miles). It leaves from the lodge.
In the morning, we did the 4 mile hike up Mt. Scott. A little bit more strenuous (but still a moderate hike) of a hike than Garfield Peak, but gives a bit of a different view of Crater Lake plus 360 degree views of the valley surrounding.
Of course, our trip wasn't complete without hiking down to Crater Lake itself! If you're brave, you can swim in the water. There are rocks to jump in as well. Do keep in mind the water is very cold, in summer the surface level reaches 50F, however, below the surface temperatures stay just above freezing year-round. The hike down to the water is just over 1 mile, however it is steep.
You can also book a boat tour around the lake, which is fun if you have the time (and money), but not a must-do. You can also choose to get dropped off at Wizard Island (the big island in the lake) to hike around for a couple hours.
We chose to drive the 50 miles around the rim of the lake on this day. There are lots of short little hikes off the road that are fun to do along the way. Sun Notch Trail is a great one to view the Phantom Ships - another small island on the lake that, well, looks like a ship. Plaikni Falls is a two mile trail that hikes to a nice waterfall. Watchman Peak Trail is another 1.6 mile trail that gives a different view of the lake that I also enjoyed.
Near Mazama Campground, there's a moderate 2 mile hike called Annie Creek Canyon Trail. It goes along a little creek through a meadow, and is a nice way to round out your trip.
You may see people up at Crater Lake with big backpacking packs that look like they've been out in the wilderness for a while. These are through-hikers who are hiking the Pacific Crest Trail. They often stop by to stock up at Crater Lake and enjoy the views.