I have always liked Discussions A-Z Intermediate published by Cambridge University Press. It has lots of interesting topics for discussion; with follow up written activities and ideas and guidelines for the teacher. The only thing you need to add to it (in my opinion) is a bit of vocabulary work to give students the language they'll be using during the discussions.
I suggest you start by looking at that. I you think it's too difficult for you pre-intermediate students, you can adapt it. It you think it really is unsuitable, please write in again and I'll be happy to give you more ideas.