I'm a big fan of solo bar meals. Anecdotally and informally, I'd say at least 1/3 of the time a stranger or group will initiate conversation with me. Overall it turns into an active social occasion for me >50% of the time.
Even without active social interaction, I find people watching to be very enjoyable anyway.
I wish that were the case in the places I am having lunch at - do you actually mean by bar some past 9pm location etc? But I am frequently at coffee shops and met so many really interesting people. Definitely a great way to connect with people outside ones established social circle.
I'll commonly do weekend lunch early-mid afternoon. I live in a highly walkable neighborhood and day-hour happy hours are common in the area I live, so there's usually decent traffic at this time.
Otherwise, dinner at fairly standard hours - somewhere in the 6p-8p range, later in some areas.
Even without active social interaction, I find people watching to be very enjoyable anyway.