This was the title of one of the sessions I went to at the Midwest Homeschool Convention. It was given by Carol Barnier and despite the issue with poor sound, I gleaned some good ideas.
First things first: Homeschooling is efficient! We are doing more than we know each and every day. So when you have a day when things are going well and you aren't going to be able to do what you planned: make the most of it.
Be creative: do an art project, make a mini book (favorite animals, favorite plants/flowers, etc.), create your own opera, sewing or baking day
Be playful: play a board game, backyard Olympics, math game, freeze tag (have them answer a question correctly in order to get 'unfrozen'), have a 'backwards' day
Be inspirational: Movie day (historical, compare historical accuracy), "Heroes of the Faith" Day--dress up like the person, act out his/her story, watch a movie about them, Nature walk with sketch books
So, when you fall off your lesson plan, pick an idea and have fun!