After a quick bus ride (typically about $15, no need to book ahead of time) with plenty of mountain views, arrive in Split, one of Croatia's most beautiful places. Hostel Split offers a variety of accomodations, including apartment style rentals, and its just a quick walk from the main bus stop.
Check out the center city by first visiting the Diocletian's Palace, a gorgeous villa so historic its registered by UNESCO. You can spend a good chunk of this day wandering around the old city and soaking in the culture and architecture.
You can spend the evening at Adriatic Beach Bar, where you can enjoy the ocean without all the crowds that accompany June and July tourism. Rent a lounge chair while the staff bring drinks to your chair, for just a fraction of what it would cost on the other side of the ocean in Italy.
Start off your day with a hike to Marjan, a picturesque park at the top of a hill overlooking the city. You'll get great excercise, a great view, and some fresh air at the top. On the way down, you can stop at one of the many bistros offering delicacies like squid ink pasta, a local favorite. You also should try some of the local gelato.
Later in the evening, for a fun night out, try the Tower Pub Crawl. For a low price you'll get incredibly friendly tour guides, drinks at every stop, and even pizza. It also includes drinking games and a t-shirt, and you'll be able to skip the lines at many hotspots.