I have some experience doing this during the day time in a megacity (over 10 million people).
Picking a location
I look for these little havens for daytime sex during first dates (fire escapes in buildings where everyone uses the escalator and/or elevator is my go-to) I found it helps if you choose a haven that's not just isolated but the place surrounding it has very few people at the time you want to use it. QUIET is a huge plus so that if you hear something you can pack up and move on quickly and your girl will feel more at ease.
My game
I think I might be in the minority here but I only do a little bit of physical touch while getting investment ("hey your hands look cold let me feel them") and very little sexual escalation during the date. Actually I don't even talk about sex... I find for my personality (more innocent and adorable) it completely destroys the atmosphere if I make ANY sexual comment or overt sexual move. For your game you can adjust I guess but I've read articles here suggesting it's a bad idea to be upfront and obvious that you are going somewhere to fuck and I tend to agree.
Her frame of mind
I want the girl I'm with feeling like she's with an adorable somewhat sexy man who's very safe to be around. I also want her thinking she needs to work a bit to hold my attention or I will bore easily (which I do). So complying with my spontaneous idea to do something naughty like having sex in public will seem natural.
Logistics
I like to walk my girl from place to place while making small talk. To transition into the haven when we get close I'll say, "hey let's go X place for Y reason," the haven is something I don't acknowledge-- it's just a place we are passing through on our date. Once we get to the haven I'll slowly stop and pull my woman back to me. I'll look into her eyes and slowly go in for a kiss. If she's receptive I start physically escalating rapidly and try to get her handling my cock within... 5 minutes or so? Always woman handles cock before I use my hand or tongue on her pussy. I'll be sure to help her out by undoing my belt buckle though
Even though this seems insufficient, I have found as long as it has a spur of the moment feel the women usually are VERY receptive to my escalation even though we hadn't talked sex at all prior. I don't fuck with upper body clothing at all by the way. Pants and underwear are the only things that get pulled down, and only just enough. Again this helps make an ostensibly risky situation feel safe enough to actually do because we can both fix our clothes in 1 second and be on our way.
There's lots I left out but that's my experience in a nutshell.