math challenge activities or program?

    We use TERC Investigations math program and are finding
    that some kids who have met the K Spring math
    benchmarks in September are not being challenged. I
    would like some ideas or programs to challenge these
    kids. I really don't want a bunch of worksheets, but
    instead some engaging problem solving math activities or
    other ideas to extend their math thinking. Does anyone
    know of a good book or other resources or programs that
    are easy to implement? Thanks!