do NOT use play sand!!
there are only a few types that are really safe for a saltwater tank such as oldcastle and southdown. yardright too. but they are still not the best as far as buffering goes like an argonite sand. most play sands are not actualy sand at all, but it is just mined. therefore, they contain all sorts of other minerals that you do not want in your tank such as silicates, phosphates, and even some heavy metals.
i also would not recomend gravel or crushed coral because it is really easy for debris to be trapped in it. even if you were to vaccum constantly, unlike a simple freshwater tank, a marine tank contains much more rock on top of the substrate (generaly) so you would only be able to vaccume a small portion anyways.
