The improbably-named Fifi Colston brings us this helpful bit of magic- turning bubble wrap into blisters. I love finding little gems like this on craft sites that normally specialize in projects best described as "precious" and "adorable". Heh.
If you're the type that loves to go over the top you could even make these poppable. Just inject it with fluid using a syringe and then seal it up with a dab of liquid latex. The best budget "goo" for that kind of thing is probably cornstarch and water with a few drops of food coloring.