Crossword puzzles are a great (and fun!) way to improve your vocabulary.
Many of these puzzles are themed, so you can learn a lot about American culture.
TIP: You can always print them out and do them with pencil and paper. If you find that too challenging, you can do them online and get hints, maybe a few letters to help you think of the word you are looking for.
It's always good to keep track of the new and useful vocabulary you learn from each puzzle, maybe by adding it to your vocabulary.com lists or your vocab notebook. And remember to use the new words! Use 'em or lose 'em!
Here are a couple relevant for SUK2 students.
Here are a couple relevant for SUK4 students.