Usually, a BBQ, or something like popcorn and soda stands, are pretty popular. You can hold the event at a community center or a park. We like to use the community center AT the park

Hand-outs including crime prevention are good things to do, as well as fingerprinting for the kids. If your department can afford it, bring in a tactical unit, K9 unit, air unit, etc., and do demonstrations.
You can also assign a couple of officers to drive around the housing tracts, say hi to the folks, do the popcorn bag/soda thing, etc. Or a combination of all of the above.