I have taken my family to Boo at the Zoo for several years now.  We have a blast.  The price is reasonable and well worth it.

From the Cleveland Metroparks Website: Boo at the Zoo is a neighborhood of safe Halloween entertainment, with a spooky delight on every corner including animals on exhibit each evening. Happy haunts include: The Creepy Crawly Critter Animal Show, the Monster Mash Dance Party, the Not-So-Haunted Greenhouse, the Jack-O-Lantern Express Train Ride, the Halloween
Hay Maze, magic shows, jugglers, stiltwalkers, musical entertainment and much more. Each child receives a complimentary treat bag. Kids and adults are encouraged to wear fun (not scary) costumes.

This year the event is held Oct 20-23 & 27-30 from 6-9 pm.

Tickets are $8 for ages 2+, buy them during the day at the Zoo Box Office or online ($1 extra charge)