Pronounce a word or short sentence with a spelling that you want to test them on and get students to write it down in their homework. They then have to convert each letter into a number, add up the numbers, and shout out the total. If they have the right total, that must mean that they have spelt the word correctly.
This is obviously also good for numbers practice, but can also be used for words that they already know how to spell but have problems recognizing due to minimal pairs etc. You can also use it as a lead into homophones.
PDF version for easy printing: Spelling code game