I need advice on how to be "platonic"...?

I've dated one of my best friends before, but we're still pretty close. However, he has suddenly started acting like we're dating again and continuously flirts with me even though I have no feelings like that for him whatsoever. He asked me to hang out today, and I can't avoid him or ignore him, because that's just plain rude But I don't want to do anything that gives the wrong impression. Where would you suggest we go/what would you suggest we do to avoid awkwardness?