During creation, many of the days have the phrase "vayar Elokim ki tov" after some aspect of the creation on that day. The second day does not -- the Stone edition mentions a Rashi that the term reflects things that are completed/final and the waters of day 2 were not finished until day 3, so day 3 has the phrase twice (which begs a different question for another day).
But Shabbat also lacks the phrase. While there is no creation on that day, there is a verb which indicates some setting aside (Vayekadeish oto) which would establish something which could be judged as "Tov". The establishing of the day as one of rest is a completed status for the generations (permanence is the required element according to the Ramban reported in the Stone) which could at least have been recognized as "hinei Tov."
Is there something in "Tov" which demands a creative act? Why can't the sanctification of Shabbat be considered good? Do any meforshim discuss why the term "Tov" is absent from the establishing of Shabbat?