I use json4s (which happens to use lift-json under the hood):
import org.json4s.native.Serialization.write
import org.json4s.DefaultFormats
val a = Array(1,2,3,4,5)
implicit val formats = DefaultFormats
println(write(a)) // [1,2,3,4,5]
It can also serialize more complex values:
case class Test(map: Map[String, Int], arr: Array[Int])
val t = Test(Map("one" -> 1, "two" -> 2), Array(1, 2, 3, 4, 5))
println(write(t)) // {"map":{"one":1,"two":2},"arr":[1,2,3,4,5]}