I agree with 25... my W and I are still together though quite troubled. We did just this on Sunday, went to an orchard/pumpkin farm with our two youngest. I did invite her, I did it in person, and kept my expectations very low. In fact for her to go meant some logistical effort on her part so I fully expected a no, but got a yes. And I think it went well...
BUT, and a big but, we're still together. I partly invited her because it would seem odd to the kids for me not to, and we're trying really hard to keep stuff normal for them. I think if I was in your shoes I wouldn't invite her. I would just go and have a really great time with my son. Let my son go tell her what a great time we had. Granted, your son is 3 so that might not work as well on second thought (mine is 5). But you can certainly send him home with "his" pumpkin for mom's house, maybe some apples or something apple-treat related there for mom's house, etc...
Let her see what she's missing out on. Let her see that you're still going to have a life and be a dad with or without her.
If you do invite her I'd do it in person or over the phone, but not by text. That way you can control the message and thus in person is even better. But you have to be sure to control the message so you're not clingy, needy, or desperate. Just sort of "hey we're going on Saturday and thought you might want to come along. No big deal, just let me know".
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD