putting them into groups and monitoring (i.e. walk around and make sure they actually speak English!) is good. Don´t let them get into the same groups every time.
1.Try debates. Give them a subject with one group preparing PRO arguments and the other CON. Ideas on how to do it and what topics to use can be found at
ESL Conversation Topics — Debate
The more controvoversial/outrageous the subject, the more fun the debates!
2. Party Games like Taboo or Apples to Apples are great for conversation classes
3. On the net, there are various ideas for ESL printable board games (mainly to teach grammar but as long as they´re talking...). Print enough copies so that groups of 2-3 students can play. Make sure you have enough pawns and dice.
Good luck!