If your cat is not fully harness/leash trained yet, I would not take it to a baseball game, way too loud and too many people.

Both my kitties are leash trained, Sammy more than Dudley. We take Sammy a lot of places with us, we can't with Dudley because he gets EXTREMELY car sick. We took Sammy with us last year to the Findlay Pet Talk Meeting in Ohio, he loved that. We also take him up to the cottage and he comes out in on the boat with us fishing. He also frequents the pet stores now and then too. I wanted to bring Sammy with us to Tennesee to the mule festival, it's a good thing we didn't, there were more people than I expected there.