For a spelling challenge that is fun and educational, Word Wise Free for Android is top of the line. There are over 15,000 words to be spelled, and you only have a short time each round. Accuracy is important, but speed is just as important. Start with 100 seconds, and gain a little more time with each correctly spelled word. If you are stumped, feel free to take the "pass" option, but know that it will cost you. Lose a little time with each "pass" you take.
Spell correctly and fast, and you will find yourself at the top of the Swarm leaderboards. These aren't just any old leaderboards either. With friend and message options, you can socialize as well as show off your spelling prowess. There is no rule against using aids such as the internet or a dictionary, but they will slow you down. Use them with caution!
You could kill those spare moments in life with any mindless game, but why not have fun and sharpen your spelling skills at some time. Keep your mind sharp and practice something useful while you wait for the dentist, for dinner to get ready, or for you kid to be done with practice. Let your kids play to keep them occupied, and feel good knowing they are learning too! It is the best of both worlds.