I wasn't all that impressed with Metalkova; it felt like an alternative shopping mall rather than a squat. The kids at the infoshop were nice, though.
Places to check out outside the Schengen Area:
Bosnia is very easy to enter and exit (if you have a western passport), and the hitchhiking is easy as well; nice, hospitable folks, little suburban sprawl, and almost every road has shoulders rather than curbs. The ability to read cyrillic will help you with navigation in Serbian areas. I would also advise one not to hitchhike at night, and be very careful of wandering around overgrown fields and forests as many, many landmines still remain. If you want to take photographs of ruins, be respectful and don't aestheticize another people's tragedy.
Croatia's nice, the coast is beautiful, and Zagreb seems to have a cozy little radical scene. Split's a beautiful city, but pretty damn boring if you're there for more than a day; there's no easily located counter-culture. The rest of the coast is very scenic, and the coastal cities up towards Slovenia seem to have an active squat scene. Be aware that Croatia has a large and active neo-nazi presence.
I've never been, but friends have told me that English teaching gigs in Turkey are relatively easy to come by, and this can be a good way to wait out your 90 days. Wages are low, so don't expect to make much money.
Kosovo/Kosova is friendly, but didn't seem to have much of interest to travelers. I'm told hitchhiking is doable here, but when you can take a bus across the whole country for five bucks, why bother? Macedonia didn't feel too different from Kosova, but was more ethnically diverse.
Montenegro is gorgeous, but one of the more difficult places in the Balkans to hitchhike. The country looks small on maps, but can take a really, really long time to travel through, as road conditions aren't so good, mountains are abundant, and switchbacks - rather than bridges and tunnels - are numerous. The coast is pretty touristy. Small, off the map mountain towns can get a little dicey at night, and after dark in border towns, you can easily be robbed, so don't stick around.
Serbia is rough around the edges, but worth a visit, anyhow. Only the intellectuals and merchants speak fluent English, so communication can be tricky. Russian is a good second language to know. Hitchhiking is easy, just know where you're going and remember your "hvala". Even with the economic reforms, Serbia still has the feeling of an old time socialist society. Novi Sad is a nice, quiet university town that has a friendly infoshop. Beograd is big and intimidating, but it's not a bad place to make some new friends and go out drinking and dancing.
Construction sites are abundant in every Balkan country (real estate's a great way to launder dirty money...), and most make excellent places to sleep on a rainy night. If you enjoy different music, bring a portable radio so you can listen to all the various types of music on the Balkans. Every ethnic and national group prefers a slightly different type of music, and different musical instruments are associated with different regions, so each country's radio stations have their own particular flavor. (Turbo-folk reminds me of ranchero...) Between Zagreb and Thessaloniki, don't expect to find much of a punk scene.
Didn't go to Albania, Ukraine, Russia, Moldova, or Belarus...
Have fun.