Dubrovnik to Montenegro

I have heard of traffic delays at border crossings, but nothing that would prevent you from getting back the same day. To me, it would be too long in a car (2 hours each way). Will you have a car or be doing a private tour?
You might consider staying in Herceg Novi, which is right over the border and only 30 minutes from the airport, same as old town.

I haven't done that as a day-trip but had no issue with a public bus from Kotor to Dubrovnik (and points north) in 2015. The drive around the Bay of Kotor is truly spectacular, but I'm not sure how much the driver gets to enjoy it, so for a day-trip I'd prefer to let someone else do the driving. I don't like to label places "don't miss", but if I didn't have that hesitation I'd but the Bay of Kotor in that category.

I did also like Herceg Novi, which gets far fewer tourists than Kotor, but if you don't go beyond there you miss most of the bay.

The Kotor (town) experience--as of 2015--was massively affected by whether there was a monster cruise ship in port.

Last week I spent a few days in Dubrovnik, then crossed into Montenegro for the night; went Herceg Novi and then as far as Kotor that day then back to Perast for the night.

I have stayed there before and I love the place (Hotel Conte).

Next day drove to Restaurant Zora on the twisty mountain road about half way between Koto and Cetinje; Its a prosciutto smokehouse with great prosciutto.

Visited Cetinje, then went to Tivat to look at real estate (crazy idea), then back to Dubrovnik by way of the ferry crossing of the bay.

Bottom line: Crossing the border was a snap.

Thanks all. We plan to go by aviator tour from Dubrovnik to Montenegro. Hopefully not too much time in a bus! If anyone has taken this tour please let me know.

Or you can do a private tour for about $250.