You're in a restaurant
Running into your ex while you’re dining out is a surefire way to spoil your appetite. If he and his date haven't spotted you, there’s no need to force an interaction. Love the one (or ones) you’re with and try to enjoy the evening without drawing too much attention to yourself. Even if you’re on a date with someone unconscionably attractive, don't flaunt it. Once there's eye contact, however, some sort of acknowledgement is necessary. A simple nod or wave may suffice, depending on the circumstances.
But let’s say you’re alone, drowning your sorrows in a plate of carbs and bottomless glasses of wine, only to discover your ex is also at said bar, having a grand old time. Don’t run! Instead, pretend that you left your credit card there last night, and ask the bartender to check. Et voila! You don’t look like a coward, and for all your ex knows, you had such a great time the night before, you left without paying.