Marlboro has outstanding recreational facilities--tons of programs for kids. There are numerous parks as well. My Gym, Gymboree, Party Gym, and others are also nearby for classes and parties. Music classes, dance classes, private lessons, etc... are readily available.
I think you posted another thread like this one. I think you'll find that most of the nice towns in Monmouth all have similar access to stuff for young kids. If one doesn't have something, you just go to the next town over.

I have done Gymboree, My Gym, and Honey Child Music with my soon to be two year old.
In the summer, Dorbrook Park in Colts Neck has a splashground that is really fun for little kids. My friends and I take our kids sometimes, and they absolutely love it. They also have a beautiful playground right next to it, with two areas--one for little kids and one for bigger little kids.
There is no shortage of stuff to do. In terms of determining exactly which town to live in, I'd take a drive around to get a feel for the areas and see which one you like best. It isn't really like, "Wow! Marlboro has X, Y, and Z," but Middletown only has "A and B." All of the towns are family-friendly.