Ludwig said:
Take a spare tube, pump, allen keys, wind up lights, mobile phone, water bottle. Fit a carrier rather that carry a rucksack.
Definitely carry a set of allen keys as well as the other items mentioned. Last weekend I was riding a forest MTB track full of twists, turns, dips and jumps and ended up twisting my handlebars. If I hadn't had the allen keys with me to re-align and tighten them I'd have had a very long and uncomfortable walk back to the visitor centre
I use this
clip-on saddle bag to carry spare tube, puncture repair kit and allen keys, and a
quick release handlebar bag to carry purse, phone, waterproof jacket, snacks etc whilst out riding.
Other essentials are gloves (I use fingerless atm), padded shorts, base top and windproof/waterproof hi-vis jacket.
Almost forgot the most important thing - a helmet!!!