Time4Learing.com was a great option for DS6 last year. You can change the grade level available very easily and the activities are fun. Kids can skip around to topics that interest them. It costs $20 a month and no long term commitment. You also have access to science, social studies, and language art activities.

We used the program for around 10 months before DS6 got bored of it. I would definitely recommend it. I think they also still give you a free trial.

Crisc