If you've only got 5 days outside of Croatia, don't try to cram too much in. I'd have suggested Slovenia, but if you want to meet people you may find it too quiet. Rome is a good call. Agreed that Milan sucks, don't bother with it. Budapest and Krakow both great shouts. Romania is also great, you could spend a couple of days each in Brasov and Sighisoara. When I went, Romania was more of a B&B place than a youth hostel mecca.
From pictures I've seen, Montenegro looks amazing. Probably low on fellow hostel-dwellers, though.
To get fucked up and meet people and just chill out though, Cesky Krumlov in southern Czech Republic is the place to be. Real nice, medieval town. Go inner tubing and pub crawling during the day, eat giant skewers of meat at night. Nothing better.