Water for butterflies
In the past, we supplied a shallow pan filled with sand and water for butterflies. We didn't notice any butterflies attracted to this, though.
We haven't done this since we've had ponds—especially our wildlife pond, since it has a gradually sloping area at the edge that exposes mud.
Any butterfly that needs the water (or the minerals dissolved in it) would be able to find it in these more "naturally"-occurring sources. Still, we haven't yet noticed any butterflies taking advantage of this.
Once, on a trip to a state park, we saw large groups of male swallowtails at a gravelly beach. It was maddening that I didn't have my camera with me, but it was fascinating to see.