A.W.
We just had my daughter's 4th birthday today- we went to Pirate's Cove in Elk Grove Village. We had a BLAST! It is a great place for kids her age. There isn't a lot for adults to do but we had a great time watching the kids have fun. It's $9 a kid for non residents and parents are free. And if you go to Oaklee's guide's webiste they have a dollar off coupon. They also have combo passes for the Pavillion's aquatic center and Pirate's Cove. That way the adults could have some fun too. There are places at Pirate's Cove for picnics too. My daughter LOVED having lunch on the top of the pirate ship. They also have a concession stand where you could purchase lunch.
Or like some one else said Chuck E. Cheese is another option or maybe a park and a picnic or a pool party at a local pool.
Good luck and happy birthday to your little one!