Asking someone to lunch or coffee in an informal way is all about being friendly and approachable. Here are some casual ways you can do it:
"Hey, I was wondering if you'd like to grab lunch/coffee sometime this week. What do you think?"
"Hey, want to take a break together and grab a quick coffee/lunch? My treat!"
"I'm craving some good coffee/lunch, and it's always better with company. Care to join me?"
"I've been meaning to try out this new coffee shop/restaurant nearby. Want to be my partner in crime?"
"Do you have some free time today? Let's catch up over coffee or lunch."
"Feeling hungry/thirsty? Let's ditch the usual and go grab lunch/coffee together!"
"I could use a coffee/lunch break. Want to escape the daily grind for a bit with me?"
Remember to keep it light, friendly, and considerate of the other person's schedule. If they decline or are unavailable, be gracious and open to rescheduling.