May is when the reservation system starts for most WA SP, so you should check out the dates you think you'll be there and see if anything is open. Weekends fill up fast, but during the week, you can get lucky..
We can reserve up to 9 mos ahead and all the best sites go fast! Maryhill is a popular SP, so if you want hookups, you have to reserve.. You might get lucky and get a standard site by just pulling in, but every time I've gone there, it was "CAMP FULL" signs upon entering.... ;) I take I84 on the Oregon side to get there and just hop the river to get to Maryhill..
If you like Astronomy, the Goldendale Observatory is pretty cool and the other sites mentioned will keep you busy for sure!
It can and does get windy there, so be prepared for that..
Mitch
