Simulations and role-plays are very useful for speaking activities, where the emphasis is on fluency and not so much on accuracy. They can be fun and your students could really grow to like them if you adapt them to their age and level of knowledge.
Firstly, let's refresh our knowledge of the difference between simulations and role-plays. When it comes to simulation students speak and react as themselves, but the group role, situation and task is imaginary. In role-plays, on the other hand, students are given a situation plus problem or task, but they are also allotted individual roles, so they are not acting as themselves, but as though they are someone else. The most important thing for both is that students imagine themselves in a situation outside the classroom and use language appropriate to this new context. You can give your students a variety of roles (profession, status, personality, attitude, mood), variety of physical settings, variety of communicative functions and purposes, which all lead to varied language.
Here you can find descriptions of situations great for starting simulation activities. Let your students prepare a short dialogue, but make sure not to let them read from their notes, they should only help with the preparation. These situations are hilarious if you are just the observer, so they should really have fun.
When it comes to role-plays, the easiest thing to do is to give them different settings, professions, moods and situations, so that they can really act it out. They could be the judges on the X-Factor, who have to tell a person that he/she really sucks at singing, or Angelina and Brad in their home trying to find all their children who are playing hide and seek, or even Barack Obama addressing public after being re-elected president of the USA. Just think outside of the box and keep in mind the aims of your activities. And have fun, of course.
Do you like simulations and role-plays? Which are better according to you?