Spider web game for rainy days. Use painter's tape to make the web and have the kids throw wads of paper at it to see if they can get it to stick.
20 Awesome Nerf Games to Make and Play (Frugal Fun For Boys)
If you did the face masks on the green balloons in blue, purple, red, and orange it could easily be a Teenage Mutant Ninja Turtles party. Sindelar PK still like Turtles?