If you're looking for some good old-fashioned fun, see if an exhibition or carnival is on at Orange County Fairgrounds. All the fun of the fair awaits!
You're spoiled for sightseeing choice when in Costa Mesa. Orange County Fairgrounds is a great place to kick off here, but there are loads of other worthy spots to appreciate. University of California, Irvine, Huntington State Beach, and Huntington Beach Pier are not far from the outdoor exhibition area and will fill in your day nicely.