If it's a close friend I'd approach them from behind and do something silly like poke them on the opposite shoulder or grab something from their hand (that's not their phone, wallet, or similarly important). Then I'd talk to them as I normally do.

If it's someone I know reasonably well (old classmate from college or colleague) I'd approach and call out their name in a clear but not too loud voice; "Hey [name], how's it going?" and ask some basic questions like how they're doing, what they're up to, etc. Mostly to show that I care enough to acknowledge them, because while it might be a short term pain for me, it can help a lot in the long run to maintain these connections.

If it's someone I'm acquainted with but don't know too well I'd pretend not to see them, maybe even slow down a little to create distance. This could also apply to the above if I'm really not in the mood to talk.

I recalculate my route to avoid being spotted.

I don't often have the ability for spontaneous reciprocal conversation while I'm also dealing with all the light, noise, temperature, and loud/bright moving objects of outside though.

Do you merely know them? Are they a friend? An enemy? Your arch nemesis?

Attempt to avoid, fail to avoid, blurt out "heyhowareyouyeahgoodthankshowareyou", walk away and think about that interaction for the rest of the day.

Speed walk and check my pulse. Say hi quickly and then speed walk off with vigorous left to right hip motion.

Jump in the nearest Wheely bin and hide under the rubbish.

Just raise your voice a little bit and say "Hey TheirName"

I wave silently and choose a route so that they could walk with me if they want, or not.

brief eye contact and give a little nod

Give them a pat on the head