What can I do to make my little sister happy and playful when I babysit her?

Today, I babysat my little sister, which was sort of great, except for the fact that she was a total monster. Every time I asked her if she had to go to the bathroom or prompted her to get dressed in clothes for the day, she would do the same things, which were of course lying on the floor while kicking and screaming. I've never seen her so upset and unruly. What I think is odd about it is that we did every single thing a little girl would want to do, seriously (we went on a hot-pink scavenger hunt outside), but she still was so mean. Even my Dad said she was being positively evil (he happened to come home while she was in the middle of one of her tantrums) and that she was acting the worst she ever had. What else can I do to make her happier while I'm watching her?