You will need some sea water.
Can you think of a way to measure the salinity of the sea water? There are many possibilities to figure out how much salt there is in sea water! How would you go do it?
When you fetch your sea water, take a little more than you need and put it in a bowl or glass somewhere where it can sit without being disturbed (maybe on top of a cupboard?). As water evaporates, what happens? Check occasionally on your glass with sea water, and look at how much water vanished since you last checked. What is happening in the glass? At what point can you notice salt crystals forming? How long does it take until all the water is gone?