Spare kit bulbs and fuses (from Halfords, who do lots of different kits so you can get the one that's specific for your car). Very useful if plod pull you for a busted light that you hadn't spotted. Should get a pleasant chat at the roadside while you change it. At least, in TVP and Surrey areas it seems to work. Probably get an ear bashing and a ticket from the heartless Durham mob.
There's a toolkit with some basic spanners and screwdrivers that sits inside the spare wheel, and I know they all fit the nuts and bolts I'm likely to find since Honda put it there.

I keep the key for the locking nuts on the alloys in a lockable compartment inside the car though.

but I forgot to unlock it for the garage when I put the car in for new discs and pads.

Bottle of oil, fuel can and some water rattling around in the boot too.
Because I'm too lazy to use a footpump to inflate tyres I keep a small compressor that plugs into the lighter socket. Just to be on the safe side I have one of those aerosol cans of instant tyre fix too. Oh, and while we're on the subject of flat tyres, though this isn't something that goes in the boot - find out where the jacking points are on your car before you need to use them, and remember that they're easier to find in the dark if you have a torch handy.
Breakdown cover is a must. Last time I broke down the dizzy had keeled over, and I was a long way from home. I don't know how much it would have cost me to get a recovery vehicle out for it, but it would have been a damn sight more than the 70 quid or whatever I paid Green Flag that year.