Interesting/important topic (everyday experiences are always good, as are topics that students are always interested in: Mars, aliens, comets/meteors, Big Bang, black holes, etc.)
A variety of types of questions
Answer format: multiple choice, fill in the blank, calculation, read a graph, free/paragraph response, plot data, draw a schematic, “discuss with your groupmates”, etc.
Presentation/group format: individuals, group, as a class, demo-related, etc.
Applies both within a worksheet and week-to-week
Active participation (get students up and moving and talking, writing on the boards, presenting to the class, etc.)
Promoting the idea of group activity (e.g., difficult or tedious for a single student to do, but can be naturally broken into independent components for different individuals)
Separate activities/examples for separate groups to compare/contrast at the end of section (e.g. build up an H-R diagram)
Be visually engaging: make your worksheets look nice (include pictures, picture captions, fill-in-the-blanks, etc. but don't make them too cluttered)
Relate concepts to commonsense notions and everyday experiences (if possible)
Use of real or realistic data and images, instead of clearly fake examples, as long as it doesn't excessively complicate the activity
Symbols are defined and used consistently with elsewhere in the course
Be relevant to homework/exam questions (so students feel it was worthwhile to them)
Be creative. Non-standard worksheets can work well. Try things that aren't discussed here - and let us know if they work!