What do you have the authority to do and/or how much to spend? For example, can you get them to authorize giving away gift cards? So say ..
"Come in and test drive and we'll give you a $50 gift card."
No doubt that will generate some foot traffic. We did that to the extent if we could tell a buyer was interested, but needed that last little push to come in that night we would say ..
"Come in the next 24 hours and we'll give you a $50 Target gift card"
We had them leftover from some other promotion. So it worked out for the best. Honestly the more money and/or bigger the prize/gift the better. We once gave away a bunch of LCD TV's. Generated a heck of alot of calls and traffic -- Though not one person actually bought a car from us lol (that was the GM's idea).
Oh, another cool one we did was .. "Find the Sonic". We drove the Sonic around to different places in the area and snapped pictures of it and posted it to Facebook. The person who was the first to respond with the location got the prize. Kind of cool, but nobody actually purchased anything from it .. that we know of :\