So many places are worth seeing:
Croatia, Montenegro, Serbia, great beaches and scenery, especially MOntenegro. Croatia is very similar to some Italian resorts.
Slovenia is great too.
Estonia is worth visiting, Tallinn is an amazing city, and there is some nice countryside in Estonia also.
Budapest is also fantastic, get there now before its taken over by stag partys like Prague has. Once you've decided I can probably tell you some good stuff to see and do etc.