Paint (we used tempera)
Sponges (we used makeup sponges but regular sponges cut to a smaller size would work fine)
A large surface to paint on (we used two large table clothes) you could use a sheet, newsprint, cardboard, etc.
We used those little sponges for makeup that are shaped like a triangle for applying foundation. Soaked them in paint and swatted them with fly swatters.
Watered down paint (we used liquid watercolors) but you could use watered down tempera paint
Surface to paint on (we used a white table cloth) You could use a sheet etc.
We also put liquid watercolors in spray bottles and sprayed a hanging white sheet. Both were messy and fun!