We're using ACE Word Building. Very independent. Still uses phonetic principles and applies them to other words outside of the list for the PACE. Words aren't too challenging, but it isn't just a list, all teaches vocab, word parts etc. Where it meets your requirements: independent (except for any questions, self tests, tests), colorful (my girls love the character building cartoons), thin workbooks (broken into 12 books/yr). Cons? not the cheapest course, I think it's around $30 in the US, we pay more here in Canada.