We just introduced DS4 to soduku. The easiest ones are 4x4s, and they help with logic. You can print some freebies here: http://www.printactivities.com/Kid_Sudoku_Puzzles/Kid_Sudoku_Puzzles.html.

I just got an add in the mail for Highlights Top Secrets Adventures, and think they might be fun for "afterschooling."

We do have a Kumon workbook, but it requires the students to write words/numbers to answer problems. You might visit your local bookstore and see if there are any that are multiple choice. (Maybe ones geared toward standardized test prep.)

Good luck...and be sure to post if you find anything good! DS seems to be similar in development.