So here are a couple things we've done recently.
Milk, Food Coloring & Soap:
This is so easy and tons of fun. Kept MiniM.E. entertained for about 2 minutes per batch... was hoping for 15, but maybe we'll get there...
Fill bottom of container with whole milk.
Add drops of liquid food coloring.
Swirl toothpick in dish soap and watch everything move!
This took longer to prep than I expected and requires a mixer, which MiniM.E. did not like because of the noise. But it was worth it for the 30 minutes of play time in our master bath.
Mix 1/4 cup water, 2 Tbs. dish soap and liquid watercolors in a mixer, dump into big container, play where the mess is contained.