There're lots of speaking activities, but to be honest role-plays are my favourite.
Role-plays, or simulations are one of the ways we can ease students' transition into using
English in real world situations. A simulation is where students act out a real-life situation, for example checking into at a hotel, but do not act out a different personality. Role-plays are where students take on different personalities. In a role-play, for example, one student may be asked to take on the role of "an angry neighbor" which is out of character for the student.
Role-plays are interesting, memorable and engaging. In their assumed role, students drop their shyness and start thinking in English.
What is important for a successful role-play? Preparation and a "hot" topic. The thing is it's not that easy to find a good role-play. And I'm always happy when the course book offers something really good, e.g yesterday we had a nice time with Business Result role-play (Unit 6 Solving crosscultural problems)
Role-plays can range from 30 minutes or one hour to a year-long corporate simulation for business English. Staging role-plays can be challenging for a teacher, but is also great fun. After you have done a few, you will know what to expect and feel more confident.
BTW I found a really great resource on role-plays. It's a presentation with ready-made roles:)http://dl.dropbox.com/u/4392166/Methodology/Role%20plays.pdf