What to do if my ex-wife put my daughter in a dance class on my visitation day without my consent, cutting my day shorter?

You have two different options.  The first is just to not take your daughter to dance class... there is nothing in the decree which requires you to do so.  If your daughter is understanding, this might be the easiest and cheapest way.  If you feel like it's putting you in a bad position with your daughter (i.e. dad's a party-pooper), then you can go to court and request a modification of your visitation schedule or the rules regarding visitation.  Explain to the judge that you would like to arrange activities that you and her could do together to foster your relationship, and her conflicting scheduling is impeding the already limited time that you have for that relationship.  Pitch two options to the judge and/or your ex-.  Option number one is ask for an order which prohibits her from scheduling any extracurricular activities during your periods of visitation.  Second option, have a fall back that if you she does schedule visitation on your time, that you be awarded additional visitation time some other time in the week to make up for the scheduling conflict that your ex-wife created.  In your arguments, a strong focus on fostering relationship with your daughter will get you more mileage that bashing your ex-wife, even though she clearly deserves if for trying to undermind your relationship with your daughter.  Judges tend to blow off ex-spouse bashing, but will agree more with parents that are focused on the relationship with your child.