A few to add to Zen Scanner's great ideas:
Rate of evaporation as a function of surface area (e.g., 1 cup of salty water poured into bowl versus cake pan versus jelly roll pan).
Rate of evaporation as a function of initial salt concentration (include a supersaturated solution made by boiling water and adding salt until no more will dissolve. Bonus - I think this will also make the biggest crystals).
Can you make colored salt crystals, too? (I don't really know). If not - why not. (Maybe the sugar is an organic molecule and so is the dye, but the salt is inorganic). Or maybe you can make them colored. I don't know!
Rate of evaporation of salt solution with a layer of oil over the top - probably pretty slow, and what does that say about the local effect of a large oil spill?
pH variation during evaporation (I don't know if there is any variation!)
This would be a tougher one - but rate of evaporation in a room made humid using a humidifier versus rate of evaporation in dryer air.
What if there is a mixture of salt and sugar in the solution? Maybe you could do some sort of relative saltiness/sweetness taste test to see if one or the other precipitates out faster. Again - I have no idea on this one.
Have fun!
Sue
