Ninjago balloon decorations:
We also had a white balloon that my son drew on to look like one of the skeleton guys. We set it up on something and have them throw "shurikens of ice" (origami stars that I made for them) at it. One of the stars hit right on the tip and popped the balloon!
We made a paper mache pinata:
The cake was the green ninja:
Super simple decor on the walls. My son colored in a lego guy printout to look like the 4 main ninjas.
They played pin the staff on Sensei Wu:
I thought it would be fun to make some sort of ninja mask as a party favor for the boys that came to the party . In searching for ideas on the web, I came across this great idea to use a t-shirt tied around into a mask (from A Lemon Squeezy Home). So I got them all t-shirts from the dollar store and sent them home with the photo of the instructions found from Lemon Squeezy so they could all be ninjas.
I even dressed the baby up. :)