only take your ignition/spare key - leave the house keys etc somewhere in the car.
I have seen someone use a key safe with number padlock and lock it to the eye tow bar at the front or rear of the car too - all cars have a tow eye somewhere if you check. Surfers use that trick too since you obviously can't take a key out with you.
1) Well organised - from information on parking/travel etc on your website through to the actual race e.g clear where bag drop is etc.
2) Well marshalled with sensible water stop (eg after 3 or 4 miles not 1 mile that I had recently!)
3) A decent medal (I like them to keep as a memory although many do not - it's a personal thing).
its possible to do the above for a decent price. One of my favourite races is The Great Weston 5. A lovely 5 miler race which has around 300 runners with medal, decent T-Shirt, goody bag and organisation like clockwork all for £12. I personally don't mind paying a bit more for decent organisation - it makes the race so much more enjoyable.
As we are off piste - the Thermae Spa in Bath after the half marathon is lovely! I know in theory warm water makes the joints swell after long exercise but the warm spa waters and steam rooms are just toooo tempting.