First off, what is a rice box? I like to get a plastic bin (whatever size you have room to store) with a lid. That makes it easier to store and hopefully prevent some spilling. You then buy bags of rice and fill the plastic bin with the rice. Easy enough, right?
Take the flashcards that you will be using with the kiddo(s) and hide them in the rice. The kids love to put their hands in the rice to feel for a card and pull it out. You can then talk about the picture, what it is, how you use it, describe it, talk about what letter it begins with, etc. The sky is the limit! It's a great language activity, but can also spice up any old artic drill and practice session. For little ones that might not be using flashcards yet, you can also put little toys in the rice for them to find.