Here is an activity from "Cool Classroom Stuff" on TeachersPayTeachers. This would be great for upper elementary, middle, or high schoolers.
And here is an example of what it might look like done in fine line marker.
Storytime Anytime reads the book, How to Catch a Leprechaun, in this YouTube video. (The book is by Adam Wallace with illustrations by Andy Elkerton.) After reading the book or watching the video, you or your sub could have students draw a leprechaun trap. (You may need to remind them to build something that would be kind and keep the leprechaun safe.)
This printable Celtic Knot coloring page is in PDF format and designed to be printed on letter sized paper. It is by Literary Springs Designs.
Here is an art activity from the TPT shop: Centers and Early Finishers. Upper elementary and middle schoolers might enjoy this.
Here is a sample of that lesson done in crayon.
And here is one done just in black pen.
Here is a free writing prompt and drawing activity for primary classes from A Classroom for All Seasons on TPT.
Here is a free printable St. Patrick's Day Word Search from MPM School Supplies.
Next is a worksheet for early finishers. This one is in my TPT shop, Art Sub Lessons.
This next one would work well for SPED or preschool. You would need to cut out the shamrocks first, but it should be a successful lesson.