J.N.
If you're doing it in the next few weeks, you might enjoy the Royal Oak Farm Orchard in Harvard, IL. We just went there last week for a field trip and it was a lot of fun. They do have birthday party options which are about $10-15/person, or you could go for just a simple tour which is about $3-7/person, depending on the day and such, and then bring a cake to have in the pavilion after the tour. You can check out their website here:
http://www.royaloakfarmorchard.com/
HTH!