I want to grab a few drinks with my ex girlfriend without telling my wife first
Why do you need to hide that from the wife?
I want to be alone and I don't want her to know.
It's OK to meet with your ex, but not if you intend to hide it from wife. Sneaking around behind her back to meet an old flame is suspicious and disrespectful. What do you have to hide?
Maybe it's not cheating since I don't have feelings for her anymore. Who cares if my wife get's mad, I'm an adult.
You should at least tell your wife though. If you tell her then there will be no reason to feel guilty since you are not lying to her.