I took my son there the Summer before last. We went in mid-July and it was hot. The elevation is high, so it can still have snow in early or mid June. The lake is beautifully blue. I was born and grew up in Oregon so it's not far from some of my old favorite places.
If you go, you might decide to purchase the boat ride and take the hike down to the water level to do so. If so, purchase those tickets at the first opportunity when entering the park. They usually sell out. They are not exactly inexpensive. You will have to hike back up and it is a bit challenging. I am overweight and I did it as a middle aged man, but my son had to wait for me as I stopped plenty of times to rest. I thought it was worth it because I was with my son. Maybe not if it was just me. The scenery is excellent, but the commentary not as much. You will hear the NPS guide give some good info, but misleading info too. Crater Lake is the deepest lake in the 48 states, and it has no inlet, so it originally had no fish in it. The guide will tell you that mankind, in his evilness, just had to put fish in it to contaminate it. That's true only in the broadest sense possible. More specifically, it was the NPS itself that put the fish in and they've been trying to get them out ever since.
I can't tell you about the campsites as we did not stay in the park.
One other thing: the Oregon Caves and Jedediah Smith Redwoods aren't far away on the Southern Oregon coast and I think they are wonderful and fun.
Enjoy your trip.