Ok so here is my two cents.
First, planning one time events on the other parents weekend is a no! no!. Why? You are taking that parents right to parent away. Instead your child should be communicating with the other parent that they wish to do a certain activity ie ask "Mum(Dad), my friend Sally is having a birthday party in two weeks and I really want to go. Can I go, please?" allow the parent the opportunity to parent.
If the problem is like ours were my spouse only gets Friday night to Sunday afternoon due to distance then be prepared to switching out weekends. Remember, if you get to see your child on a day to day basis your getting more quality time with the child then the other parent.
Also set up the situation for success don't answer for your ex. It is their choice and people are more likely to make a bipartisan decision if they actually get to be an active part of the process.
There was a problem with my step daughter and her dad awhile ago, were she would TELL her dad nearly every weekend that was his that she was having a sleep over party at a friends house and would be staying by her mum because it was much closer. Well at first we thought this it was great that she was making so many friends. As it turned out this was not the case, his daughter was making plans with the same friend every weekend. Now since there is a pretty far distance between the two and there is her younger sister to consider his oldest daughter would stay home with mum so she could go to her friends house and his younger daughter would come down for the weekend. After not seeing his oldest daughter for over a month (she was 11) the lie was discovered and her father had to make new ground rules such as it needs to be a real party or special event and she needed to ask not tell him but ask him. As a result of his oldest daughter being allowed to TELL rather the ask her father it lead to her feeling like it wasn't fair and that her father wasn't listening to her. He was its just that he couldn't always switch his plans or family events around to accommodate what his daughter wanted to do without is seriously limiting the time he would have with her. In retrospect it is how it should have been from the start and would have avoided so much drama.