I'm reminded of an activity from one of the Reward books.  I think it was the Pre-Intermediate Reward Resource Pack (but could be wrong).  I recall the activity is a board game, which was pretty good, but the the idea of the prompts they use can be extended to other activities too.

Things like:

• Describe one weekend activity you enjoy and one you hate.
• Your friend is on a diet.  What are three things he should not eat, three things he can eat, and why?

I think you could have a stack of 8-10 cards per pair or group of 3, each with something like this on it.  They take turns turning over a card and starting a 2-3min conversation based on each one.  It should be interesting because it's personalised conversation and Ss can extend the conversation as they like; and many of them will naturally encourage use of the target language (eg. "I like/enjoy doing the gardening on the weekend", "He should avoid eating pizza, but it's okay to eat a tuna sandwich, and he can eat as much seafood as he likes" etc).

Another option is to have them create a story (a murder mystery or a ghost story for added interest) based on key word prompts.  You give them a mixture of both topic based words and verbs that often predece to-infinitives and '-ing' verbs.

Try it yourself first, to see if it works.

5 teenagers           decide            dark woods           empty house             broken windows         avoid        
a creepy old man             enjoy            hunting             plan           a knife             want            a hero
ended up                alone             a wolf           start              chase               escape