I usually start the lesson with a warm-up question like "What pet do you want?" They can have a small discussion with their neighbors about what kind of pet they want. Encourage wilder ideas like tiger or bear. It leads into the presentation where I introduce adjectives and ask them what they mean in Japanese.
Then I play janken with the JTE (it's best to go over the lesson together beforehand). The winner is a customer and the loser is the pet shop owner. The customer can talk about what kind of pet they want: cute, friendly, happy, etc. The pet shop owner can have fun acting with this part! They can click the 'cute' button where one of the monsters shows up. The customer can then refuse or accept the unusual pet. The kids really enjoy the acting and reactions so have fun with it!
I then pass out the worksheet and go over the phrases and monsters as a class. They are then allowed to stand up, find three friends to janken with and suggest pet ideas. Students are allowed to refuse pets if they don't like the look of them. They really enjoy suggesting crazy things and "convince" their friends to buy it.
Can't wait to use this activity. Looks like a great supplement to the textbook.