The trip to Thessaloniki from Athens takes about 6 hours and costs €45 one way and €65 return (with 10 buses per day), with one rest stop. Intercity KTEL buses connect Thessaloniki with cities throughout Greece. There are small ticket-office terminals (praktorio) for each line and separate ticket offices and telephone numbers for each destination. You can browse the KTEL website to get an idea of timetables (it's in Greek, so use Google to translate the page), or call KTEL Thessaloniki Main Terminal, but it's best to make ticket inquiries in person. The KTEL Main Terminal is on Thessaloniki's southwestern outskirts, off National Road (Ethnikos Odos). Buses to Halkidiki leave from another KTEL terminal (www.ktelmacedonia.gr) every half-hour from 4 am until 10:45 pm. All of this can be confusing, so definitely confirm both the time of your bus and the terminal from which it leaves when you buy your ticket.
KTEL Main Terminal. Giannitson 244, Thessaloniki, Central Macedonia, 54628. 2310/595400; 2310/595421; 2310/595413; 2310/595428; 2310/595435; ktelmacedonia.gr.