Essential items!

Drinks always take enough liquids with you!
Day pack preferably a rucksack with a volume of at least 12 litres and multiple compartments; possibly with pockets for an integrated drinking system and a rain cover.
Food bananas, muesli bars and other carbohydrate-rich foods are the most suitable. For emergencies: dextrose tablets.
Maps (1:50,000) even though the routes are signposted you should always take a map with you. Some signs could be missing!
Tools multi-tool for small repairs, pump and replacement inner tube, chain tool and a suitable master link, inner tube repair set.
First-aid kit crashes and minor injuries cannot always be avoided!
Mobile phone in an emergency you can call for help – although reception may be poor in certain places.
Lights and reflectors: see and be seen! Even though these are not part of the standard equipment of mountain bikes, in autumn and winter battery-powered lights for front and rear as well as reflective clothing are essential.