Silly Monsters for an Art Sub Lesson
This looks like a fun lesson. It is called "Silly Monsters" from the blog "Her Dabbles." She describes the lesson and has several child drawings from it. If you have a little time, you could come up with a fun drawing lesson for your kids to do with your sub. Have them add seasonal elements and it is evergreen for whatever time of year you want.
Spring Doodle Monsters
If you want an art lesson that is all ready to hand to a sub, check out this one in my TPT shop. Doodle Monsters in Spring. It's easy to teach and do. Kids will find it fun to do.
Inexpensive and ready to hand to a sub, this is great for when you do not have time to plan a whole new sub lesson.