Planning a one week trip through Istria (Croatia) and Slovenia

[Adar]

I am discussing this with some friends and I am looking for advice, input or reasons to convince them that my idea is good. We are going in early July and the girls want a decent amount of sun so Britain (where I was 2 years ago and loved it) and NW France are out of the picture.

Some options are:

Southern Spain: Sevilla, Cordoba and beaches is a nice combination.

Czech Republic: Beer and mountains is nice.

Croatia-Slovenia: I want to experience the Slovenian caves for a day or two and then move down the Croatian coast.

Would you prefer one of these or should we go for something completely different?