- A small pack of sticker type patches. These work great for all but the biggest holes and are usually plenty durable to get you home or back to the car.
- One plastic tire lever. The plastic levers are light and won't damage rims or tubes. These usually come in sets of three, but one is enough to get the tire off the rim.
- CO2 Inflator with 1 gas cartridge. These devices are very handy, but they're a one shot proposition, so make sure you've got everything in place before you pull the trigger.
- Folding mini-tool with 2mm and 8mm hex keys and screwdrivers. This should be more than enough to attend to most minor repairs and adjustments.
- Cold, hard cash. A useful tool if you're near a convenience store and feeling depleted or if you feel like swinging by the coffee shop after a ride.
- A cell phone. The cell phone serves as a last ditch option if mechanical problems are too big for the mini tool or patch kit. It's also advised to use the phone to alert your significant other if you decide to extend your ride.
If your rides take you deep into sparsely traveled woods where cell service is sketchy, you might be advised to pack your seat-bag something like this.
- Full patch kit with glue, sander and levers. The glue-on patches are more durable and reliable than the stick-on variety.
- Spare tube. When a tube is badly damaged, sometimes you have no other option but to install a new one.
- Hand pump. A hand pump is generally more reliable than a CO2 pump and has the added bonus of allowing you to adjust your tire pressure based on different trail conditions.
- Full featured multi-tool. On epic rides far from home it's nice to have a tool with additional options like a knife, spoke wrench and chain tool.
- Zip ties and small roll of electrical tape. These two supplies, combined with a little MacGyver spirt, can temporarily fix almost anything that can go wrong with a mountain bike.
- Picture ID and health insurance card. If you go big and don't quite make that transition, it's nice for whoever finds you unconscious at the bottom of the mountain to know who you are.
These are just a few suggestions on how to pack a seat-bag. You'll most likely create your own, customized gear combination that's based on your own riding style and the locations of the trails you ride. There is also the consideration of variable bike repair skill. Some riders might have no problem truing a wheel on the side of the trail while others might have trouble getting a clincher off a rim. The main thing is to carry whatever makes you feel confident and comfortable enough to handle any situation on the trail.