You could backpack around Europe, visit all sorts of major cities of each country. Sniff in some culture and eat local food.
You going to Poland is a great start, drink some Polish beer and eat some local food. Than you mentiond Greece, once again get some culture and eat local food and just pin point a few cities of your interest and do the same there until you go home.
So the point of this is:
Doesnt matter where you go, do atleast the following