This is a very popular activity for the Spanish class that we can use in different ways. I like it to review vocabulary and to spark conversations about topics my students love.
When the main goal is to review vocabulary, I choose several words we've seen in class and group them in series of 4. If we have time, we can use pictures instead of words, but if we don't, just write the words on the board and you're good to go. Feel free to use this example as well.
If the topic of our last classes was La casa, we could write el tenedor - la lavadora - el armario - la mesa on the board and start the activity. Tell students they need to find which word doesn't belong to the group and why.
It's up to the teacher to place a word that clearly doesn't belong (for example, three -ar verbs and one -ir verb) or just 4 random words that allow several possibilities. In the previous example, we could say el tenedor doesn't belong because it's a small object. But we could also say that la lavadora doesn't belong because it's the only one that needs electricity to work.
Check the answers students have and share all the different possibilities.
If your main objective is that students talk in Spanish, I recommend you choose words or images about topics they like. There's an example here with Harry Potter, Star Wars and The Avengers. In this case, there's no specific vocabulary to review, but students will be more motivated to explain why they think Professor Dumbledore or Leia Skywalker are different than the other characters.
We could ask students, individually or working with the whole class, to tell us why each element of the series is the different one. In our first example, we already said why el tenedor and la lavadora could be the different ones. We can now ask why la mesa is the one that doesn't belong (it's the only one we have in the classroom, for example) or el armario (it's the only one where I could hide inside).
If we work with the activity with the different characters, we could use it as a model for students to make their own designs about topics they like (sports, animals, singers...). Then, they can present them to the class and ask their classmates which one doesn't belong. This could be the warm-up activity for many classes to come, having each day one student or small group run the activity.