Hello and welcome to RTA !

On the way to the Grand canyon you would also have the option to visit Bryce canyon and/or Zion NP's. From the GC to the coast via some giant Redwoods [Sequoia not coastal Redwoods] you could drive across Death valley and detour into Sequoia NP, home to the biggest of all, the General Sherman tree. There are some smaller groves just north of Lake Isabella in the Sequoia National forest such as the 'Trail of 100 Giants'. Travelling over Labor day weekend will mean heavier traffic and visitor numbers so you might want to get booking lodgings near these popular spots.