Rice can be poured and scooped much like sand. It may not work as well for sifting and it should not be used in humid environments, such as outdoors. However, it is safe to be ingested in small amounts and is relatively clean, at least initially. This is a viable option for inside play.
Children can shovel pea gravel into a pail and dig holes much like they can with sand. A benefit to using gravel is that, unlike sand, it can be washed off if a child spills something in it and it doesn't stick to sweaty bodies or fingers. Ensure that the gravel is small enough not to be a choking hazard for young children.
For an indoor box, use small beads instead of sand. Beads can be washed if needed, and can even be sanitized by periodically soaking them in a sink or bucket with water and bleach. Use a strainer to drain the water and spread them on paper towels or newspapers to dry.
Sugar crystals are a more sanitary option than sand, and there is no concern regarding ingestion. Children can use plain white sugar in almost the same ways they use sand. For smaller "sand" boxes, colored sugar crystals may also be used. Use empty shaker bottles from kitchen spices or the empty sugar crystal bottles to pour and shake the sugar out. Monitor children closely to ensure they do not eat the sugar.