Thessaloniki is an interesting place for the traveler/activist. There is lots of international people active here to support the local economy and non hierarchical organisation.
The system of Thessaloniki buses seems chaotic for an outsider. There are no maps or timetables on the bus stops. Just electronic signs with coming buses. Buses stop for a short while and sometimes ignore people. And it's nearly impossible to guess which bus to board to get to your destination. (Maybe try to ask around?)
You can get some info at the company's website: http://oasth.gr/en
It's easy to use buses without a ticket. Supposedly, if control approaches you (which is rare), you can say you're foreigner, refuse to show your documents and get off at the next stop. (I haven't experienced that.)
They're digging a future metro now (2017).
Lots of potential squats and a starting up platform http://elefterospiti.weebly.com/
You might be able to camp in the university as cops aren't allowed in. Just keep an eye on your belongings. Cops are allowed in now (December 2017).
dumpsterdiving is very easy. Ask bakeries at closing times or during worktimes for some leftovers and you might get even a bag of things.
A quiet place with internet and electricity is the public library at the corner of Alexandrou Svolou Street and Ethnikis Aminis Street (here). Go to 3rd to 5th floor to connect to the internet.
Busking seems to be allowed. You can go to Aristoteles square or elsewhere in the center. Local jugglers sometimes go to perform at the traffic lights to earn more.
Radical freespace: Agriokata at KostiPalama 12. Here you can exchange anything you like, have free WiFi, food and intersting people who support the alternative life. (Dec 2016)