Personally, whether road or mountain biking I like to have the following
Spare tube
Mini Pump
Small repair kit including, chain breaker, spare link, multitool, patches.
Latex or nitrile gloves x 2 pairs. One for working on the bike, one for the first aid kit.
Small first aid kit -gloves as above, plasters, steri wipes, sling bandage, painkillers and a space blanket. Keep it all in a ziplock bag.
Head torch.
Small cable ties.
Little hotties hand warmers.
Money - don't forget money. I keep a £20 and some assorted change in a small ziplock.
Mobile phone.
All of the above (except the phone and pump) will easily fit in a mid sized saddle bag. the pump can go in the rucksack if i'm carrying, if not it'll go in the jersey pocket.
If I'm heading up the mountains or out all day where I need to bring a rucksack I'll bring some extra food in wrappers. flapjacks, snack bars, chocolate and a raincoat. Maybe a small flask of coffee if it's really really cold.