This is what's in my backpack when I go out mountain biking. Tools, spares, food water and a first aid kit. Whilst it may seem like overkill to carry this much on any ride I tend to treat all rides the same in terms of spares and tools. With the exception of very local rides where I may be prepared to walk home I always go out equipped for self sufficiency, I've had too many mechanicals to go unprepared. Even keeping your bike maintained and serviced there is no stopping that stick going through a wheel or that rock smashing into a derailleur.
A basic first aid kit is a must for me, it's not just for me and those I'm riding with you never quite know what situation you'll come across while out in the wilds. So having decided that the kitchen sink needs to come along how do you carry it all?
There are a few options for carrying all of those spares, tools and food. A backpack, this is my current option, I have a Camelbak Mule. A bum bag is another option, these have become popular of late as they leave you back free. Alternatively you can strap spares to your frame using a number of strap or bag options. I use this method to carry my spare tube as it makes my pack lighter, the downside to this is if you have multiple bikes you have to remember to swap spares and tools over.
You will need tools to get you out of any mechanical issues, basics are a Pump, Multitool and Tyre levers. You may also want to add in a CO2 inflator in case you need to reseat a tubeless tyre, a Leatherman or Swiss Army Knife can be useful for more advanced trail side repairs or if the worst happens preparing first aid dressings. A chain tool, Spoke key and a Valve insert tool can be handy too, this does depend on what multitool you have as to whether you need the additional items.
Having the tools to fix an issue isn't much help if you don't have any spare to use. A spare tube is always a good idea even in the age of tubeless a split sidewall isn't going to hold air or if a Tubeless repair kit fails to fix it. I carry a 27" tube on the grounds that it can be made to fit 26"-29" if needs be. Valve inserts can be handy surprisingly I have had one fail on a ride, luckily it held.
I carry missing links for my chain, mainly because I went through one chain that just kept breaking and ended up more links than chain. I also carry some spare chain links, leftover from shortening my chain.
A Gear cable saves single speeding it home, along with Mech hangers for your frame and derailleur. Cable ties and Insulation tape can be used to fix any many of issues so are always handy to have. If you have a reverb seat post an Enduro height collar can be handy if it decides to fail on you.
It may sound a little overkill to carry a first aid kit and thermal blanket round the local trail centre. However if you adventuring further afield and into the mountains, I would toughly recommend carrying at least a basic kit You never know when you may need it, even if it's to dress a cut caused by a simple crash, or to help others.
Food and Drink
No one is going to be with out some trial snacks, whether that be homemade or commercial available. I'm not a particular fan of gels although I keep a couple in my backpack for emergency use, I do prefer real food such as flapjack, fruit or even crisps. On a long ride I would probably take a proper packed lunch, but on short rides a chocolate bar stuck in a pocket is enough. Jelly babies are another choice as they are tasty and packed with all the goodness you need.
On the hydration front I use a mixture of water and electrolytes with or without carbohydrates depending on the duration of the ride. On all day rides where I may need to top up my fluids I will carry some hydration tablets to make up drinks on route. I will use the bladder in my CamelBak but only for plain water, but for electrolytes I'll use a bottle.
So that's what I generally carry I'm not saying this is a definitive list, it's been added to or amended through experience over the last few years. It has also been influenced by being prepared, a mantra instilled in me by my dad who was in the Mountain Rescue, a service I hope not to have to use.