Southeastern Idaho, Central Idaho, Northern Utah, Southern Montana or Western Wyoming
My wife and I are leaving the kids with Grandma and Grandpa for our Anniversary and going camping!
Yes I live in SE Idaho, but I've only been camping here for two season so I'm FAR from well versed on a lot of the "spots".
I was looking in Western WY but it appears most of the stuff I've found like Gros Ventre is still closed. Coulter bay we can only stay in the parking lot.
If I find a place with hookups thats reasonably priced great, but far from necessary for three nights of camping. We'll be getting out and exploring the towns that we're near too.
I was going to suggest Craters of the Moon, as we really enjoyed that a couple of years ago. Unfortunately neither their campground or the private campgrounds in Arco open until the beginning of May.
Perhaps you should consider delaying the camping portion of your anniversary celebration for a few weeks so you have more options.
Echo on the Craters of the Moon and Arco area - but you could also move on down to the Bear Lake area on the Utah border. I think the Logan Canyon area has many USFS campgrounds. We're not big fans of the Idaho State Parks website, use this instead: http://www.bearlake.org/camp-pic.html
Not a fan of KOA either, but they're nice and Bear Lake KOA opens April 1st.