Sandboxes are fun both outdoors and indoors. When used outdoors it's good to put a sheet of plastic under the sand to prevent your tots from digging through the sand and into the soil which will make the sand dirty. It's also great fun to flood the sand box during warm weather so slope your plastic so that the water will drain away from the house and toward a shrub. Pierce a few holes in the plastic on the downhill side so it will drain slowly.
When used indoors I recommend putting your sand box in the middle of a tight weave area rug or carpet remnant. This way you can just lift it up and dump most of the spilled sand back into the box. You can also vacuum the carpet if you want to be extra tidy or take it outside and throw it over a clothesline and beat on it. I prefer carpet rather than vinyl because carpet tends to clean the toddlers' feet and trap the sand whereas vinyl keeps the sand right on the surface to be tracked all over the house.