we should visit slovenia in juin. What are the most interestings things to see or to do?

You will hopefully have good weather in June to start with.

Slovenia is small, but packed with diverse scenery: you'll be able to enjoy the scenic Alps, the Adriatic coastal towns and the beautiful green valleys. Bled and Bohinj are the two lakes that many consider the emblem of Slovenian tourism, Bled being a mountain lake with a castle on an island. But so are the Alps and I like Bovec more than other centres. You'll be able to see Ljubljana, the capital, in a day or two - but it's cozy and charming despite being small. Try not to miss Piran and Portoroz on the coast. Ptuj is also an old town well worth visiting. If you're into sports, you'll find there are a lot of opportunities for kayaking and camping in Slovenia.

Slovenia is promoted by the Slovenian Tourist Board:

http://www.slovenia.info/

And the Slovenian Government:

http://www.ukom.gov.si/eng/slovenia/in-brief/

BTW In WWII it was occupied by the Germans, Italians and the Hungarians.

The most beautiful place I think is Lake Bled. There is a castle overlooking the tranquil lake. The Julien Alps in the horizon. Also there is a fantastic mideval church on an island in the center of the lake, amazing! breathtaking views. This was Tito's vacation home. The people were also very down to Earth and a pleasure to be around. Hope this helps. Also further south are a wonderful cave system called Postjana, One interesting fact, The Germans never gained control over this region during WWII, says something about the resilence of the people.

I can also highly recommend the Lake Bled area´, Bohinj, Ljubljana and of course Portoroz, in fact the entire Slovenia is a hidden gem most people don't realise!! Oh, and you can also take a catamaran trip to Venice from Piran!