A few come to mind:
1) Long trip, young guy and girl. They're drunk. They're cool and we're chatting. Then things get very quiet. There's a tension in the air. I hear a belt buckle klink and I know what the deal is. Just some mutual masturbation, but they didn't finish (I don't think). The guy gave me a good tip when we got there (not like that) because we both knew what was up.
2) A regular. She was an exotic dancer, and she was cool AF as well (most of the time). She had just got a boob job. She showed the new pair to me. They were nice.
3) Another couple who were regulars, with an extra dude along for the ride. These were rough, tough, street people. Anyway, we get to their destination and the girl and the new guy get out of the car and cross the street, but tell me to leave the meter running. The bf stays in the car, but something doesn't feel right. I ask the bf what's up, but he just says that he's not involved in it. After a couple of minutes, they come SPRINTING around the corner, jump in the car and yell "GO GO GO GO!" I do it, because I have no idea what we're running from, and I don't really want to die. It turns out they just robbed a girl of about an ounce of crack. I'll never forget the girlfriend telling the story about how she hit the other girl over the head with the handle of her knife and then yelling, and I quote, "MY DICK IS SO FUCKING HARD RIGHT NOW!"
4) Another regular. She's a young, highschool-aged girl. Her and her friends to TONNES of drugs, and she makes it very clear that she can get me anything I want. She's cool and nice. One day, it's just her and I in the car. She tells me that she has Cystic Fibrosis and probably only has another couple of years to live. That's why she just parties all the time and doesn't give a fuck about school. She hasn't told any of her friends, because she doesn't want them to treat her any differently. I'm one of the first people outside of her immediate family that she's told.
That one was heavy. I moved away soon after, but I hope she continued to live right up until she died. :)