Often in smaller classes you may only divide your classes into two teams for games and activities, but if you have larger classes you're going to have to divide your class into groups. In smaller classes it's easy to practice one on one using one person for from each team to compete. Pairing individuals to compete one on one in a large class is not efficient or practical. In large classes that is replaced by making groups compete against each other.
Some of the activities here will require some preparation and others no prep. Here are some examples of activities and games that work well with groups and large classes.
Warm-up Activities and Icebreakers
Elementary-Adults. For this warm-up activity and icebreaker you will need to have each group stand, pair with another in the group and stand opposite one another. Then they will ask each other get to know you questions. You can watch it here.
Kids-Adults. This activity works well for teaching vocabulary and spelling. It works well with small and large classes. Depending on your resources it may require some prep. You will need a set of letters for each group. These you can find in a stationary store. Letter magnets are also a possibility or you can just make them yourself from index cards.
You will need to write a letter on each index card (cut in half) and you will need doubles of the most common letters. Each group will need a set of letters. If you are short on time you could have each group help you make them. Paper is not recommended since it will get damaged. You can watch it here.
Kids-Adults. This is an efficient vocabulary activity where students get to practice speaking, writing and listening. If you have a large class of say 40 students you can divide the class into four lines of ten students each. You can watch it here.
Games for Sentences/Q&A
Elementary-Adults. This activity for sentences/Q&A will work well in groups and large classes. This one is similar to "Make a Word" above. However, instead of a word we will make a sentence instead. You will need index cards or scrap paper for this. You will then take a sentence from your lesson and write down each word from the sentence on an index card or piece of paper. You can watch it here.
Elementary. Power point games similar to this will often work well with large classes and groups. Instead of pairing one person from each team pair the group against another group. Have the group say the sentence or question and the fastest group to say it correctly wins. You can watch it here.