Each of the girls has one workbox designated as "extra reading." I've put three or four books in each one, and they can choose which one to read. Crayons is supposed to read hers for at least 20 minutes a day (although she often goes longer) and Ponytails has a 30-minute reading goal.
Speed-reader Crayons has already finished The Mouse of Amherst, Jungle Doctor's Monkey Tales, and is working on Happy Little Family. I may have a problem keeping her in books that she hasn't already read.
Ponytails is working on Farmer Giles of Ham and is planning on starting A Wrinkle in Time after that. Or maybe My Side of the Mountain. I have always been a bit uncertain about L'Engle's books (for theological reasons), but I thought Ponytails might like this one.