vendeavours said..
Valet key is the go if you can get won Be careful with key safe's you are advertising that there is car key inside ,they can easily be cut open with battery tool It has happened to friends
Maybe get a keysafe and then stash your key under the bumper. Thief comes along and tries to crack it only to find its empty/whatever twisted thing you want to leave inside.

I guess the downside there is they might smash the car up in anger though?

I had a bag 'tidied away' once by council staff. Topped off an already bad day (had a bit of a mare, lost my board and had a 20 min walk back up the beach). Would have been very stressful if I couldn't get back inside my car. Managed to get reunited with the board and bag but all the bits and pieces that were in the pocket of the bag (couple of carabiners and a leash for launching, line extensions, puncture kit, chocolate bar) never made it back.