Another vote for the Dalmatian coast. Especially if you can rent a car and just drive around. Split is beautiful, although a bit heavily-travelled. I really enjoyed Pula, on the Istrian peninsula up closer to the north. The driving along the coast is really beautiful, and if driving is not an option, the ferries are terrific. For a bit less touristed area, the northern coast of Montenegro is also really beautiful (and a bit less expensive).

LJU is great, but I would vote for Croatia and the coast.