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.
There are a few refuges where you can stay overnight for €15 per person per night. You can also camp.