Topeak waterproof saddlebag, with puncture repair kit, tyre levers, spare innertube, spare powerlink for the chain, multitool, CO2 inflator & spare CO2 canisters. Never rattles at all!
I have a pump, spare gear cable, tool for getting tough tyres back on the wheel rims, spare seatpost clamp, two more spare innertubes, 2nd repair kit, spare multitool, cash, mobile, bandages, plasters, antiseptic cream, handwipes and Sudocreme and my packed lunch in my rucksack (a pretty small Deuter Race EXP Air)!
That way, if I pop out for a short ride without my rucksack, I have enough kit on me to get me going again. (I've had a number of gear cables and seatpost clamp bolts go on me over the years, hence the reason I carry spares!!). I keep more in my rucksack as I have it on every day during my 20-mile round trip to-and-from work, where there's more time for things to go wrong (and further to walk to get home / get help).