I recommend taking a spare tube, patch kit, tire levers, multi tool with allens and screw drivers and chain tool, seat bag, Pump, Light, gloves, Helmet, liquids, energy bar. I also have a $20 bill in my bag most of the time just in case.
Where do you store your stuff? I have friends who love their Camelbaks. I don't disagree with that but I do not like to drink from a Camelbak. If you ride both mountain and road it is best to have a Mountain Camelbak and a Road Camelbak. Well the big thing is the tubes are different. A friend had his Road stuff in his bag and couldn't fix his mountain bike flat because he didn't have the right size tube. If you ride with Murphy you realize when you are improperly prepaired problems will strike.
I use a seat bag to carry my Mountain Bike Tool Kit and I have one on each of my bikes. I don't like to get caught without my basic repair kit. Yes I have duplicate tools but it isn't that expensive.
I was recently on a ride with a friend of mine. We had done a bunch of climbing when my friends bike started the skip every second revolution of the crank. His chain had started to come apart and the link was pulling apart. He pulled out his Crank Brothers 17 Tool and fixed his chain. In less than 5 minutes we were back in motion. I have never seen a field chain tool work as fast as this. This is a must have item in your Mountain Bike Tool Kit.
I used to be a little more of a minimalist and I didn't bring a spare tube. I would just bring a patch kit because I do not get many flat tires. I was near the halfway point on a 20 mile out and back ride. We were riding along a ridge that goes to the overlook at the Dead mans point. We had climbed about 1200 feet elevation. As I came down the lastdown hill before the halfway point in the ride I hit a rock at 30+ miles per hour and pinch flatted. Before I could stop my tire was completely flat. Once we stopped I discovered that my valve stem had pulled out of my tube. I only had a pump and a patch kit, no spare tube . I spent half an hour filling my tire with grass. It was better than walking but not much.
I wrap mine with a plastic bag or part of a cut up innertube. If it bounces around in your seatbag it is possible for holes to be rubbed into your tube. Check out your tubes regularly--every couple hundred of miles. It is no fun putting in a spare tube only to have it go flat instantly. I was watching a race in 93 or 94 with Julia Furtado solidly in the lead probably 5 minutes ahead of second place. She flatted right in front of where we were sitting. She pulled her spare tube put it in and gave it air from her CO2 quickfill. Her spare tube had a hole. She put her second spare tube in and it had a hole also. She had to quit the race because she couldn't legally borrow anybody elses stuff and both her spare tubes had worn holes in them.
I was on a ride with a friend and we decided about half way through the ride to take the long way around. We realized a little bit later that we were out of water. It was really dry on the way back. We lost all of our zip because we were so dehydrated. Always take more water than you think you will need. It is painful to run out. We were nearly in trouble by the end of the ride because of a simple lack of fluids. Take a lot of WATER!
I have always used a seat bag to carry my little tool kit. It is just big enough to carry my small assortment of stuff. Always zip it back up even when you know you are going to put stuff back into it before you ride off. I have taken tools out of my kit and not zipped it up because I knew I was just going to put stuff back in. Instead of putting the chain tool back in I just left it with my friend. My zipper was unzipped and all the rest of my stuff was scattered somewhere along the trail before the ride was over. We went back looking for it and didn't find any of it.
Sometimes things go wrong. If you are planning on getting back just before dark sometimes it just doesn't happen. One night on a group of six ride we had nine flat tires. Yes it was dark before we got back to the cars. I now leave a Cateye HL-EL530 on my handle bars all the time. I will take it off on an early morning ride but if you have ever had to ride out in the really dark night without a light you won't want to do that again.
You won't break near as many lights if you mount them under the bars like in this picture.
The most important of all Mountain Bike Tool Kit takealongs--
Friends and if they are bike mechanics all the better.