Olympos National Park
There is a bus from KTEL Macedonia on the west end of Thessaloniki to Litochoro on the east end of the National Park.
By foot or by donkey.
You can buy a map of the National Park at kiosks in Litochoro that is one side a geographical map and the other side a touristical map showing only pathways through the mountains.
Days' Walk from Prionia down to Litochoro Travel to Litochoro and from there take a taxi up to Prionia (25 euros one way, approx. 25 minutes drive). Spend the day walking back from Prionia down to Litochoro (approx. 6 hours walk) along Trail E4 which is a single path all the way and is clearly marked.
Stop to see the waterfalls along the route and visit Agios Dionysios Monastery (free - clothes to cover up are available at the entrance).
There are a few refuges where you can stay overnight for €15 per person per night. You can also camp.