Please be prepared as Belgium weather can be very unpredictable (also in summer months). It is possible to have all types of weather and nights can be cold.
Comfortable backpack or trolley
Hiking shoes or comfortable trail shoes
Sneakers and/or sandals
Wind and rainproof jacket and pants
Comfortable clothes for hiking
Sleeping bag (comfort > 10 C)
Suncap, sun lotion and mosquito spray
Warm clothes for in the evenings
A small daypack is useful but not obliged
Water bottle
Headtorch with needed batteries
Personal toiletry such as e.g. toothbrush, soap, towel
Personal medication
Swimming suit and towel (a must if you want to enjoy the lake)