Hi there,
I've spent quite a bit of time in Montenegro - October should be a really nice time of year to be there! I'm assuming you will have a car? I do have a few thoughts on your itinerary.
Perast is lovely. As a matter of fact, it is where we usually stay when we are in Montenegro. Be sure to visit the little islands in the Boka, if you can.
Herceg Novi is larger, of course, with a nice old town. One of the nicest things about Herceg Novi is getting there, because the drive will take you all around the Boka - lovely views.
When you go to Lovćen, you will be quite near Cetinje, a very important place in Montenegrin history, and it is definitely worth a visit. I think it is a very attractive city.
Sveti Stefan is now a hotel and offers very little access to those not staying there. Sveti Stefan, in my view, can be best appreciated from afar. If you are landing in Podgorica and driving to the coast, you will have some really nice views of it along the way. While you probably won't be able to actually visit Sveti Stefan, (who knows? maybe they've changed something...) there are many lovely promenades right along the coast that you should be able to access at different points, between Sveti Stefan, Miločer and Bečići. You may have to ask around.
Budva is lovely (although I can't help but remember it in its pre-earthquake, pre-mass tourism days!). Be sure to walk all around the city on the old walls, and you can also follow a walkway out along the shore to have a beautiful view of the Budva Stari Grad promontory jutting out into the Adriatic.
Another longer day trip that I can recommend (if you have really nice weather and you're up for some driving) is to head inland following the Morača canyon towards Biogradska National Park (hiking, lakes). There are spectacular views along this drive which follows the Morača river and brings you past the lovely Morača monastery (definitely worth a visit) and through the small but very sweet town of Kolašin.
Well, those are just a few of my thoughts. Have a wonderful time in Crna Gora!
KateN