Lessons about secular holidays are meaningful for any age group. For younger children, K-5, each time period can be dedicated to a different aspect of the holiday. For example, Presidents Day can have implications for math, social studies and English. For older students, you're supervising a single subject area, so focus on printing up poems or short stories, math exercises, or government or history lessons for your temporary students.
The five senses can provide lessons for almost any class, from biology to English. For younger kids, you can make it a class activity or a pretend game show, while with older kids you can explore how science explains our amazing senses, or how poets have expressed what they perceive through those senses.
Older students are capable of using critical thinking to analyze current political, social, cultural and relationship trends, especially if they're given some help in discerning fact from opinion. Some excellent conversations can take place with junior high and high school-age students. With younger kids, it's a little harder, but they aren't completely unaware of what's going on in the world. You can feel them out with some general discussion and try to learn their questions and fears, while not scaring or burdening them with too much information. For grade-school kids, current events lend themselves well to artistic activities and expression.
Games are a great way for kids to have fun and for teachers to sneak in some learning. In addition, learning games for any age could make you the class's favorite supply teacher. An Internet search for learning games, or a browse through the regular teacher's shelves, can reveal fun, age-appropriate games that you can use with the kids in your care. You can end your supply day confident that the students enjoyed your class and learned a lot.