Hi Kasei,
A few years ago I backpacked across Europe myself, to this day it's still one of my fondest memories, I had an amazing time as I am sure you will yourself, if you go to Belgium, I highly recommend you go to Bruges, it was my favourite place out of everywhere I visited, its one of the most amazing and beautiful cities/towns I have ever seen and I hope to return there sometime soon. It's one of the most preserved medieval places in Europe, the Town Hall for example has never been knocked down or refurbished, simply added on over the centuries, so it is this one building that changes ages in architecture and design as you walk around and inside it, bizarre but amazing and everyone is very friendly.
If you do end up going to Greece, it's very hard to get to, but there is a tiny (TINY!) village on the south coast called Stoupa I stayed at for a week, its so small the taxi company only has 1 driver, and regularly gave free lifts as everyone knew each other, again everyone was friendly and it is the most isolated, peaceful town i have ever visited, hidden behind mountains, if you do go there, visit the bakery along the coast, you'll never want to leave.
I hope you enjoy yourself wherever you choose to visit!