Obviously you will have asked for a tank of fuel! The other things which we have found invaluable are the rubber bump strips on the doors which stop those idiots who open their doors on you in car parks! Even if you can't get them as freebies, it's worth paying for them.
We blagged half of the merchandise display in return for promising to write a good review of the garage. Not sure what we will ever do with a mini Peugeot backpack (with some strange character on it) but my 10 year old was delighted at the time!
Free mats and at least half a tank of fuel are a minimum requirement in my opinion.