If there is a set child visitation between two parents such as every other weekend, can or should a father use some extra opportunities to visit the child, such as "swing by the house on his lunch breaks" or pick the child up after he gets out of work to take him/her to the park for an hour? Is this OK? Legal? Stable?

Sure, if it's OK with the custodial parent.

As noted, if the mother/custodial parent doesn't object to it, I don't see what the harm is.

Who thinks it's "unstable" for a father to spend extra time with his child?

Im not sure I would do it without having a court order. There are any number of things that could go wrong if someone wants revenge. If the parents agree then it shouldn't be a problem for the courts to put a stamp of approval on it.

The parents can not see eye to eye on terms and will soon hopefully get a set visitation order, this is why the question is being asked because there seems to be a lot of animosity coming from the mother (custodial parent). Currently the father struggles to get the child on weekends as it is.

Yes, my worry is that with the mother acting the way she currently does that she could get upset one day as you said and in spite put a stop to these undocumented visits which would then leave the child hanging and that's where the stable part of my question comes in. None the less if there's no legal barrier I guess there's no harm in trying to spend as much time as possible together.

My current partner was in your exact situation. My advice is to ask the mother to see your child as often as possible, but do it by means of e-mail or such that it can be documented that you did ask. Have her respond to you in the same manner. That way it's documented that you do want to see your kid for more than what is currently allotted, and if she does decide to be hostile or uncooperative, it will be on record. All kids going through a divorce will struggle... it can be made easier or harder depending on how often they see BOTH of their parents. So yes, it is for your kid's benefit to stay as involved in his/her life as much as possible. My partner's ex is very hostile, and he still (after 3 years) struggles to see his kids even on court-ordered visitation. It's so incredibly sad, because she's only thinking of herself and her needs, and putting their kids last, and they are paying for it in such a big way. His case ended up being evaluated by a psychiatrist and they had a custody assessment completed. He lost, and one of the reasons was that his ex claimed that he never showed any interest in seeing his kids after they split and because he had no proof that he had desperately tried to see them, she won. That's a blatant lie... he tried many times, just as you said, stopping by on his lunch hours, stopping by after work, constantly phoning her and asking to see the kids, to have the kids as much as possible, and she kept refusing. Finally, on advice from his lawyer, he started putting all of his requests in writing. But it was too little too late. Had he done that from the start, he would've had sufficient proof that she was withholding his kids from him and the court would have ordered more visitation or reversed the current custody arrangement. Don't underestimate the power of a vindictive, hateful ex. Cover all your bases for the sake of you and your kid. Good luck.

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As noted, this is all up to the court order. I suggest you try to get the order to read something like visitation according to the following schedule PLUS, such times as the NCP requests providing he gives reasonable notice and does not interfere with the primary custodian's activities.

It is a shame that parents would would fight over this as the impact on the children is negative.