My older daughter is going to be turning four years old in a month and she has her heart set on a "pirate-mermaid-superhero party". It took a while to get our heads around what she meant, but I think we have is sussed now. I drew her a pirate-mermaid-superhero to go on her party invite and we printed out a few copies of the black and white line art for her and her little sister to colour in at home. Here's what a pirate-mermaid-superhero looks like...
So, first thing is first. We need some mermaid tails! I went to goodwill and picked up a gigantic pair of old sweat pants that looked to be a suitable green colour and a similarly gigantic gypsie skirt in lighter green with silver trimming all around it at three levels. I didn't think to take a pic of the garments before I made the first tail, so here they are with one leg and the bottom section of the skirt missing. You get the idea.
I sewed one level of the gypsie skirt to the end of the bottom half of a leg of the sweat pants and then threaded in some elastic for the waistband. I had some very very old and slightly congeled silver fabric paint that I painted the scales on with, but it seemed to work ok.
The kids really like it. The littlest one insisted on wearing it for most of Friday. I have enough material from the two garments to make four mermaid tails I think.
This is something that is quick and easy and that I didn't think of trying until recently because I assumed (as it turns out, wrongly) that it would be too expensive and my kids were likely too young to get the most out of it.
There is a Japanese store near us called Daiso and they sell many many different wooden model kits from cars to dinosaurs, to dolls house furniture and more. Each of these kits has loads of little pieces and although quite complex and fiddly, costs only $1.50 a pop.
This afternoon just one of these kits occupied four kids between the ages of two and four for around an hour. They diligently painted each tiny little piece all different colours with watercolour paints and then I fitted it together for them, to their delight. 
The kids painted the castle pieces first and then while they were drying we painted the base with brown and green and a nice blue moat to surround the castle.
Then everyone ate lunch and ran about like they were deranged while the base dried.
We glue gunned the castle pieces together and then the kids played with a load of toy animals and cars in the castle (I think we might have to have a project where we make knights and peasants and a king and queen to actually go in the castle, because as entertaining as it obviously was, driving cars around the moat and storming the castle with a giant polar bear is neither historically or geographically accurate.)
Each shoe was made by folding a piece of letter sized printer paper. My older daughter has got size 10.5 feet and we managed to make shoes for her just fine. Much bigger than a size 11 and you'll have to use newspaper or anything you can find larger than the letter size though.
It took 4 staples to secure each shoe. You could do it with three though if you had a decent stapler, rather than a teensy weensy cracker toy one that I've got. She had fun painting her slippers and then putting stickers on them when they were dry. I made sure that we put stickers over where the staples showed on the inside of the shoes, so they wouldn't scratch her.
