I don't think Rabat is one the usual tourist trail but it is worth a day or two. There's a good mix of places here that are part of the official WHS listing.

The mosque that was never finished is fascinating - it looks like it could be ruins but the tower is complete, which should be a clue that actually it is just unfinished.

The Kasbah is beautiful and great to explore. I would recommend going at sunset because it's at its prettiest then.

And the old fortress of Chellah is also really interesting. There's not much left standing but you can get a good sense of what it was like.

I would recommend staying in the medina and there are some really nice riads here. It is much quieter than the big medinas in places like Fez or Marrakech and you can easily spend some time exploring what it has to offer.