The easiest I've found is polyurethane ... spray it on from a rattle can or wipe it on with a rag. A few coats is all it takes.
Or you can use any oil based wood finish like linseed oil, tung oil, tru oil (which is varnish and tung oil blend I think). I've even used beeswax mixed with olive oil (heated until it melts into the olive oil, pour into jar, let cool and it's a paste). The beeswax mix smells good too.
Hard to beat the ease of use, minimal time, and moisture protection of an outdoor rated polyurethane like spar urethane.