I've done it in the backseat of a car while someone was driving.. in the middle of a corn field (I was working- clearing weeds from the field and I decided to take a well-earned break). My best friend has a shitload of weird places though... on the school bus, standing in the middle of a a crowd at a concert, on a fully-seated train, etc. I'm sure there are more but can't remember which other ones she told me. She's got guts...
Oh, and I don't know if this qualifies as a strange place but my first few tries at masturbating were when friends were sleeping over- we'd talk about sex, then about masturbating, then lying in bed we'd do it and inform each other how the process was going (although lying on separate beds, lights out, hands under the blankets. So nothing that freaky)