Yes, Montenegro is worth more than a day. I didn't have a rental car so didn't have the opportunity to visit Lovcen or Durmitor National Parks, which sound lovely, but all of the following were interesting:
- Herceg-Novi (in the north, just slightly off the road to Kotor)
- Ulcinj (in the far south, with sort of a Turkish feel)
- Cetinje (old capital, in the interior; different from the coastal towns)
- Budva (quite picturesque, but to me not terribly different from Kotor)
- Lake Skadar (birds, sort of marshy)
As has been previously mentioned on this forum, the most direct-looking route from Kotor to Cetinje is a road with dozens of switchbacks, which many would prefer to avoid. It would be easy to take a different route and still see Cetinje.
Much of the countryside is dramatic and hilly/mountainous, especially over toward Serbia. Try to get beyond the coastal towns, nice though they are.
In my opinion, the most interesting thing about Podgorica is its name. With limited time available I'd skip it.