The Edessa waterfall (Καταρράκτες της Έδεσσας) is probably the best known waterfall in Greece and is easy to reach. Actually it are multiple waterfalls and they are located near road 2 when driving from Thessaloniki to Florina.
Take the exit to Edessa and follow the signs “waterfall”. The Edessa waterfall is located at the east side of the city. I am not sure if the signs to the waterfall are very clear. There is a free parking in front of the Edessa waterfall park and until now the entrance is free.
The Edessa waterfalls are quite exceptional. Centuries ago people made a bassin for their water supply but in the 14th century the river changed it course creating first one waterfall but later on several other small waterfalls. Nowadays you still can see several of the waterfalls from a higher viewpoint.
The left (single drop) waterfall is called Karanos and the right twinfall is called Lamda falls. The total height of the waterfall is appr. 70 meter. It is possible to walk behind the big Edessa waterfall (Karanos) where you will have a perfect view with a water curtain.
At a lower part of the Edessa waterfalls you will find a cave (small entrance fee) which is of great geological value. The entrance of the cave is covered with moss and is gorgeous at the inside.
The Edessa waterfalls are the main tourist attraction in the region but is a visit certainly worthwhile. Even when staying in Thessaloniki a daytrip is possible. It is only a 1,5 hour drive (90 km). Maybe you can compare the Edessa waterfall park with the Marmore falls in Italy. These are a little bigger but the atmosphere and park design is similar.
If you want to see the full power of the waterfall wintertime or early spring should be a perfect period.