Thessaloniki, the 2nd city of Greece!
Plus points: It's cheap, easy to get there and great food and culture, the elderly are really helpful (if they speak English)
Minus points: Not everyone speaks English, some locals are rude especially teens, traffic is terrible and a cab is expensive.
If you ever go, you'll find plenty of cheap hotels or hostels, just make sure to have a private bathroom (things get nasty if shared)
Greece is known for their wild parties and loose teens/adults, so care where you go (or not if you're into that)
Either way, it's a great place to visit even if you're only interested in nature, history or and culture.