This idea was also from Family Fun magazine. Their castle looked much better than ours but this was lots of fun!
1 cup sand
1/2 cup cornstarch
1 teaspoon alum (spice section of grocery store)
3/4 cup water
Mix sand, cornstarch and alum in a saucepan with a wooden spoon.
Add water and stir until mixture is smooth.
Cook over medium-low heat until clay starts to thicken about a minute or two. Scrape continually from the sides and bottom of pan.
Remove when clay has thickened to a play dough consistency about 3 minutes.
This was a little more in depth than some crafts but worth it. We threw ours away but evidently it will harden after about a week and you can have a piece of your vacation at home!