I would absolutely get new sand. If the sandbox is an actual wooden box, you can make a cover for it. I made ours by using indoor/outdoor upholstery fabric several inches bigger than the dimensions of the sandbox, cutting out two of the corners, hemming all around, and putting in two grommits at each corner. (It also helps to reinforce at stress points.) We also put two eye-bolts at the bottom corners of the front of the sandbox, stapled the cover to the back of the sandbox, and we use bungee cords that are loops through a plastic ball to fasten the cover through the grommits and the eye-bolt. I saw something similar on a sandbox cover website. Our four-year-old can manage the cover by herself, and our sand has stayed very clean. Plus it helps to corral the sandbox toys. Good luck.