One mile south of Seaside [1] in a lushly green meadow is Circle Creek RV Park and Campground (85658 Hwy. 101, 503/738-6070, www.shopseaside.com/circlecreek [2], mid-Mar.–Oct., tent spaces $22, RV sites $35). The campground offers showers, a small store, picnic tables, and fire rings.
Other camping options convenient to Seaside include Fort Stevens State Park [3] (Astoria [4], reservations 800/452-5687, year-round, $18 tent camping, $22 RV camping, $20 yurt, $4 hiker or biker) and Saddle Mountain State Park [5] (Cannon Beach [6], 800/551-6949, Mar.–Oct., $9 tent camping, no reservations accepted).
Links:
[1] http://www.moon.com/destinations/oregon/north-coast/seaside
[2] http://www.shopseaside.com/circlecreek
[3] http://www.moon.com/destinations/oregon/north-coast/astoria/sights/fort-stevens-state-park
[4] http://www.moon.com/destinations/oregon/north-coast/astoria
[5] http://www.moon.com/destinations/oregon/north-coast/cannon-beach/sights/saddle-mountain-state-park
[6] http://www.moon.com/destinations/oregon/north-coast/cannon-beach