Need suggestions on 2-3 week trip to NW without reservations

There are those that have to have reservations and there are those of us who wing it. Like you my plans for this summer have changed AGAIN so having reservations would be worthless. Like others have said, it's the weekends when CG tend to fill up so snag a site Thursday or early Friday (Non holiday weekend) and stay put in it over the weekend. Take some yellow tape and a folding chair that you leave to reserver your site while off in the TC.
I like this site as you have the option of selecting various types of CGs... Casino, USFS, SP, and often overlooked County/City Parks. I also like this site fo public CGs only as it uses a map with links for more details.
The nice thing about that area is it's cool enough so boondocking without a big generator is easy. Most of the places we stay at are no reservations and you put your money in a steel pipe. With that small a rig you will not have any problems except on weekends if you have not found a site and then you might have to go to a private CG.
My kids were not impressed that Crater lake is one of the deepest in the US or that the things sticking up around it are the petrified cores of volcanoes. I would put Mt. Rainier and Mt St Helen above it for visiting with kids.
We were in that area last summer and other than two rallies, we did not have reservations for the rest of the time and were able to stay on the beach and on some of the rivers along the coast without any reservations. Relax and enjoy your trip.

We've gone to Crescent Lake which is north from Crater Lake for several year with our MH & boat, week long no reservations. There are many high mountain lakes in the Cascades with several NF campgrounds around them. You'll find some in Southern OR around the Redwoods too if that interests you.

I'll second what SteveAE said, it's very doable due to all the NF campgrounds.

View of Spring Campground at Crescent Lake... hard to beat the view and the price.

Hum, I have never made a camping reservation in Oregon or Washington.

That time of year, the coast is pretty busy, but if you get there early and are flexible, you are likely to find something.
In the Cascades, there are hundreds of Nat. Forest campgrounds that you will find openings in. And, if you don't, just go exploring down a logging road.

Come on up and relax.

If you want to see Crater Lake then stay at Broken Arrow CG on Diamond Lake. It's a huge CG and is never full. It's about 20 mins to Crater lake from there. Lots of CG's in Oregon don't take reservations. If you show up on a thur or friday morning you should be able to find a spot. The coast will be all but impossible to get a spot on the weekend. But inland as well as central Oregon will have spots. St parks are the hardest to stay at without res during the summer.

Thanks. Yes, we would like some mild hikes and rivers and beaches. I have read that many people enjoy Mount Saint Helen's and Rainer. Are those areas busy and require reservations as well?

Without reservations, your best bet might be to drive along and check the private campgrounds along the coast. The state parks might be a better/cheaper choice, but most will be full from those who made reservations.

The national forests offer camping without a reservation system, using a "first come - first served" policy. Most of the sites are very primitive. I would assume you'd want something to keep your kids involved, such as a river or the ocean beaches.

Good luck.