Or paint it with at least four coats of paint, on all sides. The first two coats should be an exterior primer, then two finish coats. Remember, paints now have no lead or mercury in them, and offer much less protection than paints of the past. You will also have to repaint every few years. Pressure-treated wood needs no paint, but can be painted if you wish.
Cedar does offer some protection, but even cedar is attacked if the conditions are right.
California redwood is often attacked by the eastern carpenter bee, redwood is expensive and the wood very soft, suitable more in a decorative situation rather than a structural one. Mount your trim pieces, especially stair components, kick plates and sill plates, with non-rusting screws, not nails. Resist the temptation to erect or buy one of those fancy redwood swing sets they sell. The bees will make short work of it if you're not vigilant, and the kids won't want to go near it with all those bees buzzing around, even though the bees won't hurt the kids. Forget redwood, use pressure treated wood for your swing set and paint it if you want it to look like redwood.