If you've never been to Slovenia, then you're missing out. Most people I've talked to have never even heard of the place and that was a blessing. It's got a little bit of everything, except for crowds.
Why You Should Go
A coastline along the Adriatic (for all those that like Croatia), mountains in the Alps that'll remind you a little bit of Northern Italy or Bavaria, a capital that has bars and history but bans cars from the center, and caves like nowhere else in Europe. Though, any trip to Slovenia would be remiss not to include Slovenia's most famous locale, Lake Bled.
What To Do
Situated about an hour and half east of Trieste in Italy and about the same time from Ljubljana it's not too difficult to get to (I recommend the bus instead of the train from Ljubljana, though renting a car is a great way to get about so you can see the surrounding area). Here, you can swim, kayak, paddle, or sail on the lake. You can hike up into the forest nearby or visit the castle. Along the shoreline, there are a fair amount of bars and restaurants, though they're somewhat touristy and gimmicky. Accommodations range from villas on the waterfront to hostels and campsites. So, there are places for families to backpackers.
I recommend staying there for at least a night, maybe even two. I'd start by renting a boat and paddling across the lake to Bled Island. There you can check out the baroque church that was finished in the 16th century and has a lot of well-preserved frescos and architecture from that period. You can climb the 52-meter high tower and get a view of the surrounding area. Or if churches aren't your thing, you can just hang out and fish all day on the water.
Nevermind the duck, that water is crystal clear and about 27 degrees Celsius (80 Fahrenheit). Then spend the night eating and drinking. Everyone will make you choose between Union or Lasko, so pick wisely.
Get Out of Lake Bled
The second day, you can spend hiking around the area. Bled Castle is a popular destination and only about a 15-minute hike, but if you want the best views go up Velika Osojnica. It's about an hour and a half but definitely worth it.
If you're up here, have got more time, and want to stay away from the peak season crowds then check out The Vintgar Gorge which is more impressive than the lake, but less accessible and limited to hiking.
If you're still not feeling secluded enough or want to get away from any development than you want to go to Lake Bohinj. It's got a lot of the same activities as Bled with more action sports thrown into the mix. It's definitely worth a day trip.
Lake Bled is a fairy tale locale, and there's really no reason not to go. Like I wrote above, there are tons of varied activities all over the country, but I think this is the real gem. You can spend anywhere from a few hours to a few days and still feel content, but slightly dissatisfied because you know there's probably something breathtaking that you missed. Really the only option is to explore it for yourself.