Keep it cool
It’s important that your ex thinks you have moved on, at least to a certain degree. You want to keep it cool when you run into them, definitely don’t tell them you miss them or want them back, but at the same time, you should be friendly. You might want to casually drop into the conversation what you’ve been up to. Lately, this might gently make them a little jealous about how you are having fun without him/her.