Buy the 20 gram CO2 cartridges. I heard those things can inflate a car tire.

I carry one 20 gram cartridge, a small scab patch kit (not much bigger than a postage stamp), a large freezer/Ziploc bag, and one tube. When I change a tire I put the old tube in the freezer bag. If I have a 2nd flat I double my chances of having a patchable tube (by keeping the old tube) and I'll still have enough CO2 to get by.