I am teaching similar sized classes and found http://www.designerlessons.org
to be helpful. It has a lot of material for adults and the lesson plans will help you encourage them to speak. It's good for when I'm stuck for ideas or need something different. the only thing I would say is that 30 mins is a very short time, so you would have to adapt the plans quite a lot.
If the site isn't good for your needs - try looking at onestopenglish.com - it has a lot of variety, but you have to pay for their resources.