Hi Alberto!
This is a very nice trip you are planning! There is a 5day music festival in august at Kastoria, where you camp next to the river, you might be interested in.
http://www.riverparty.org/main/

Have fun!

Irene

You can also check this out: http://www.oikotopia.gr/EarthFestival-en.htm

Earth Festival: It is an alternative ethnic festival.
It is organized between 12 to 15 July 2012 in the village of Vlasti, Kozani.

You can also pay a visit in thessaloniki, (easy access by train. I think that it will be at your way anyway) and from there, to halkidiki that has some of the best greek beaches. Of, course, if you have enough time you better visit an island! There are some beautiful small islands near Athens like hydra, Spetses etc, however if you have enough time you better vitit Cyclades islands ( http://www.e-kyklades.gr/tourism/index.jsp?context=1501&lang=en )

In addition, I would definately recomend Samothakι, which is located in northeastern greece(http://www.samothraki.com). It is an alternative island with natural beauty and many young people.
From wikipedia:
Do
Swim in the waterfalls and pools of the river affectionately called 'the murderer' "Phonias" (Greek: Φονιάς). There are more pools upstream if someone is willing to do some climbing.
Visit the hot spring/baths at Loutra.
Swim at the sea. There are 2 excellent beaches, in the west and east end of the island. The road running across the island is ending on both of them.
Visit the Sanctuary of the Great Gods and the Ancient City.

also don't forget the "gria bathra" (old pool could be a translation) (it is not in wiki, but it worths).

I hope i helped. :-P