Bubble painting envelopes = cup of water, 2 tablespoons of paint, and a few squirts of dish soap. Blow with a straw into the cup to make lots of bubbles pile up on top of the cup, then press paper to the bubbles so they pop and leave bubble prints.
Here's a cool DIY idea for a cat enrichment toy. Glue together a group of toilet paper rolls inside a tissue box and randomly fill the tubes with treats. Your cat will be stimulated when they "hunt" for a treat. So simple and inexpensive.