I LOVE silicone molds. They don't have to be expensive: I found a star mold at the thrift store, Doctor Who on Amazon's Black Friday sale, and Star Wars during a ThinkGeek sale (Though those were more flimsy). Spent less than $10 on 4 molds!
Now, you can stick to the coolness factor of ice cubes but there are so many other possibilities out there!
This month I made crayons using bits that my kids had broken while coloring. I am more than pleased with the result:
All you need to do is take off the crayon paper, break into smaller pieces if you want it to melt faster/have a more tie dye effect, and bake on a tray on low (no more than 200*) for 15-20 minutes.
Let it cool, pop it in the freezer to make it easier to remove from the mold, and then you'll have the coolest crayons ever!
Next stop... making out-of-this-world desserts! :-D