I've cycled around Croatia a little in 2010. It's very pretty there, but I would personally stay away from the coastal route, it gets very busy in summer with all the tourists. The islands are much better in that respect. Also, as soon as you head inland it gets much quieter and cheaper (but more hills to battle). If you're not set on coasts, Bosnia has lots of quiet roads and beautiful landscapes, but make sure you take spares, since there aren't many bike shops. The Danube trail is probably worth it if you want it flat. I've cycled this from Germany to Budapest, and that part is very easy to navigate and traffic free (in Germany and Austria at least). It's also quite boring in my opinion, but could be the right first trip.
If you want spectacular coastal landscapes, think Norway. It's an absolutely stunning country, very little traffic (in the North at least), very considerate drivers, good infrastructure with lots of bike shops (although distances between towns can be far in the North). It is also very very expensive. It's legal to wild camp though, and I have had good luck with knocking on people's doors to ask if I could camp in their gardens/fields. Usually got me a free shower and sometimes dinner or breakfast
Also, in Summer, you'll have a lot of daylight (sun doesn't even go down in the North in June), which is brilliant for touring.
