I'm going to say I was busy all weekend
I am not sure I understand. Why are telling others you were busy?
I don't want to explain why I was "busy".
While it is not good to break plans, it is understandable that you want to spend time doing other things. As long as you let them know so they can make changes it is okay to be busy doing other things.
I might not tell them that I was busy but instead I'll say that I was partying.
I think you can tell them the truth. If they are good friends they will understand. Everyone wants time to themselves sometimes.