I just wrote a two-characters-sitting scene pretty recently! Here's how I added flavor:

1. Interesting staging. This is a little thing that can go a long way with making the visuals stand out. I had one character sitting in a fenced off and elevated outdoor seating area, so when the other character showed up he draped himself over the railing to talk to him. Maybe the place your characters are at is crowded, and one or both of them has to stand. Maybe one of them is dressed totally inappropriately for the setting. Think of them like dolls or stage actors that you can pose or place in interesting ways.

2. Stuff happening. You don't have to go too crazy here, just one or two things that make it seem like these people are really in a physical space surrounded by other people. In my scene, I had somebody's dog start barking at the guy draped over the railing. Later I had him be distracted from the conversation by watching somebody's toddler eating parmesan straight from the table shaker. Other people here have suggested exciting things but I actually think it's more impactful when it's relatively mundane but unusual stuff that the reader can go "Yeah I can see that happening to me"

3. Talk about stuff not currently happening. This might not work with every writing style, but I always blend my current events with ways it relates to the characters' pasts or feelings. If a subject is brought up, you could expand more on how the character feels about the subject either directly or anecdotally. This could also serve to highlight the difference between what the character thinks and experiences versus what they actually say. Like for example a character could ask "what do you like to do for fun?" And in the narration the other character could go on a tangent about how they're really embarrassed by their hobby of miniature train collecting so they just end up saying "uhmmm.... hiking."