I have a son that's turning 2 on October 9th and I'm in the same boat you are. I've been to Old McDonald with Daniel during the week and it was actually fun, it's $7 per person, even for babies, but I think they have group discounts for birthdays. I think Daniel would have enjoyed it better if he had other babies he knew to play with but he still enjoyed the train ride.
The best I can come up with is having just a hand full of children at our home for some cake and some entrees. Maybe for a couple of hours, since he would be fussy if it was an all day event.
Hope the info on Old Mc Donald helps.
P.S. Those deer in the petting zoo actually bit the kids if they know you have food.