My 4 year old found a book with ocean crafts in it. She turned to one page and said "Can we make this today?" I looked at what we needed: ziplock bag, blue hair gel, Styrofoam, beads or shells. Oooh, cool, yea, we can make that and we'll use up that hair gel that's been sitting the the medicine cabinet for 4 year now! It was simple and she loves it!
First you take the hair gel and pour it into the bag. The bag needs to be full enough that when you lay it flat, a layer about 1/2 an inch thick of gel is in all parts of the bag. (We were a little short on gel, but she didn't seem to mind.) To make it easier I put the bag in a mug. While she poured the gel, I cut fish shapes out of Styrofoam. I didn't have any meat trays, but she was happy with egg crate lids so we have white fish with red and blue patterns on them.
When the bag is full of gel, you place the "fish" and the "bubbles" (beads) inside, release the extra air, and seal the bag. As a safety precaution, I also duct taped the top of the bag and made a rule that they can only play with it in the kitchen. Happy girl and her ocean in a bag!
A variation I thought of is to use homemade fingerpaints instead of hair gel. I think it'd be much cheaper considering it'd only use water, food coloring, and a few tablespoons of cornstarch. I might try that next time, but for now this worked and it cleaned out the medicine cabinet.