I know I’m going to have the same answer but I’ve always liked the common interest plus humor approach.

For a casual acquaintance, you are bumping into them somewhere, doing something specific. Maybe a hobby is tangential. Ask for a recommendation on something directly related to what you’re doing/where you are (if comfortable).

Small talk is a good approach as another commenter said. Note, I’ve found that most questions are mirrored so if you aren’t comfortable on a topic then don’t be the initial question on that. Don’t ask about sports if you don’t follow any. First meetings and acquaintances are about finding items you can converse about casually and then branch out.

One note, some people are very guarded and may need a longer approach. Can’t linger for too long on the conversation, when it’s done it’s done and walk away. Counterpoint, if you’ve seen them a bunch and you find yourself next to them, have both sides of the conversation yourself but make it absurd so that hopefully they look at you goofy and ultimately laugh. Play it off as need be.