We used any thick liquid I could find (I.e., dish soap, tempura paint, mayo, yogurt, ketchup, mustard, choc syrup, etc.). I recommend double bagging them in gallon freezer bags for durability. We added glitter to the dish soap one. The top left one is mayo with a few drops of food coloring. Tape the bag closed and tape to a window or wall with packing tape. When you're done, take down and toss!
I'd love for you to comment with your ideas. This was really fun!!!