Mlini is just outside Dubrovnik and is set up as a resort village full of quite large hotels. Located near to Cavtat, which is where you fly into to Dubrovnik.
If you're staying in Mlini, you are probably on a package holiday, and the hotels put on entertainment nightly in the season.
The sea is great for swimming in, and the area around Cavtat/Dubrovnik is relaxing and 'chilled out'.
I'd advise renting a car for a couple of days and travelling further afield though, as almo suggests, after a couple of days in Dubrovnik, you've seen all you are going to see.