You've created problems that don't exist. They're concerns that most people don't have.

You don't know what to say. That's fine.

You might be wasting their time. That's fine.

You might not know how to end the conversation. That's fine.

It's ok to be awkward. It's ok to be honest. It's ok to bother people as long as you take their feedback appropriately. It's ok to walk away without saying anything.

The more often that you talk to people and actively reflect on the REAL outcomes of it, the sooner you'll realize those concerns aren't shared by most other people.

Those forced conversations have a shelf-life because they’re artificial.

Note, rather, how friends converse and how little scripting is involved. When two good friends meet they don’t say their profession, or academic rank, or ask interrogatory questions. They exchange enthusiasm for each others presence and the conversation tends towards exchanges of perspective/experience and reflection thereof. Statement, vibes, counter-statement(?), more vibes.

That kind of familiar, friendly, approach to conversation is always available and short circuits the scripts. It efficiently probes for people who want to talk and what they want to talk about. It also tends to involve a lot of dumb-yet-charming assertions about the current situation, which takes awareness not planning. A ‘sense’ of humour, not a tight 5 locked and loaded. “Fuck, now that’s a lineup…” isn’t a refined piece of social engineering, but it’s a serviceable conversation starter and the least important part if you’re still talking three hours later.

I find myself wondering about these exact questions from time to time.

Honestly, it doesn't really matter what you say. It's mostly about body language and not seeming like a threat (smile). You can talk about whatever. Tell them about a movie you just saw and ask them about recommendations. Ask them for restaurant or dinner suggestions. Tell them about that article you just read which you found intereting.

If they are open to small talk, they will drop some tidbits that you can spring off on. Conversation is a two way street. If they don't seem interested in keeping the conversation going, tell them to have a nice day and carry on with yours.

Myself, I tend to ask with open question, a good ice breaker is: "Hey mate, hows your day going on a scale of 1-10?"

That gives them a chance to take a break and think. If they say three, you put yourself in their shoes. "Hey, that sucks. what's going on?" just lending a warm comforting subtle "I'm around right now if you want to talk"

If they give you a higher number, inquire why. "Wow, a 7? That's great. How come?" Both results in giving them option to speak. You compliment in both situations, a win win. If they return with a single closing statement acknowledge it and move on, "all the best".

If you need to end, a simple of "hey, it was great talking. I've got my coffee it was a pleasure talking to you while I waited, I've got to take off but it was a pleasure to talk".

You cater it to the situation you're in. If they want to talk then they will, if they don't then they won't.