How about a good, old-fashioned CAR WASH? A few buckets, some liquid soap, a couple of hoses and lots of old towels and employee volunteers! This would be a great way for the staff to give back to the people who rent from them... Have a couple extra $$? Hot dogs, buns, condiments, water, sodas and chips will be less than $200!
Whatever you do PROMOTE the event and make sure every tenant who comes to the office gets asked, "Are you coming to the Car Wash next Sunday?"
Finally, after the event, post some pictures from the event and thank everyone who got their car washed! That's right, part of being a great host is thanking people who attended an event that you worked your tush off for...