I've used superglue frost before with good effects. Takes a bit of practice and care to control, but you can get a nice, powdery pure white look that way.
I know baking soda mixed with glue is popular, but I've had some bad experiences with that yellowing and going fizzy while in storage, so I can't recommend it personally. I have a bottle of glass microballoons (sold as a resin casting filler) I use for snow, and it looks aces.
Some types of clear gloss sprays give a nice crystalline-looking granulated topcoat for snow if you deliberately spray them from too far away. The trick is to find just the right distance so the spray dries in transit enough to form a powdery finish, but without drying so much that the powder will just rub off when you touch it. And of course using a clear that's non-yellowing.
I'd like to try reflective paint powder at some point. Stuffs made of tiny glass beads that are supposed to reflect light in narrow angles back to it's source. Basically the stuff the reflector stripes on bike vests and the like use. Expensive stuff, but it seems like it would have a lot of potentially interesting uses.
I also keep meaning to try some experiments with paraffin wax to simulate pond or glacier ice.