I tended to use objects for the easier categories and ramped it up to verbs and adjectives for the more difficult categories. Here are some examples of the words I used:
Easy: book, car, tree
Medium: movie, snow, light
Difficult: open, shout, slow
Very difficult: surprise, clean, throw
Split the class into around 4 or 5 groups and have a student from the first group come up and choose a category, with 1 point on offer for a successful 'easy' drawing up to 4 for a successful 'very difficult' drawing.
Once the student sees the word they have 1 minute to draw on the blackboard, remembering to follow Pictionary rules such as giving no verbal clues and not drawing letters and numbers. The remaining students in the team try to guess the word (while the rest of the class stay silent).
Continue the process with the other teams while keeping track of the scores.
To spice things up a little feel free to let other teams guess the word if the original team are unsuccessful in their 1 minute, giving them the chance to take each other's points.