I have a couple things that i can remember.
When I was around 4 or something, and just entered preschool, I had this weird nightmare. Me and my mom were at a futuristic mall in the sky or something, and she dropped me off at my preschool with all the other kids that I remember were there. I go into the principal's office, and the principal was a cartoon zombie, with bright lime skin. I slam the door, but the principal just opens it again and starts walking towards me. I think this is me fearing school or something.
Another dream, when I was 5, I "woke up" in my bed. But when I look next to me, there are two tall stilts wearing pants, and I woke up screaming.
Then, when I was 7, I had what I consider to be my strangest dream yet. I was running through the sewers wearing pajamas, and I enter this doorway that says "DO NOT ENTR" (the misspelling was in my dream). It led to this abandoned hospital, where the only light was from computer blue screens, and flickering lights. It had strange posters hung up around everywhere, like a skeleton wearing pink glasses. After exploring for a bit, I turn around to my sister, in pajamas, and strangely, hold a teddy bear (She doesn't even own one). I say, "Go back, this place isn't safe." Then I woke up.
Also, I recently dreamed that I was playing LSD Dream Emulator, but that's not a nightmare, it's me dreaming about playing a game about dreaming in my dream.