I've been sewing this mermaid outfit recently for F. When we were in Vietnam earlier in the year she found this sparkly pink Lycra material on a stall in the fabric market we visited and its been destined to be a tail ever since.
It's basically a tube skirt with some pieced together shapes to make the flippers. I also got some pink net and sewed that inside the flippers to give them a bit more dimensionality - it's quite long as she wanted to be able to lie around and have her feet covered, but then she's able to hitch it up and be able to walk around.
She needed a top to preserve her modesty so I picked up some plain t-shirts and attached another bit of the Lycra to it and sewed it into the side seams of the t-shirt. I also gathered the centre so that it looked like a bikini top.
And here's both of them getting ready to go to a Halloween party. R's going to need a new costume soon as he's had the dinosaur one now for a couple of years. I'm thinking of a lion next.