After some experimentation, the winning material is electrical tape- the office dots peel up (they're re-positional which means they don't stick) however, the paper hole reinforcers are great for making eyes and they stay on the plastic eggs.
Permanent marker for details and you're good to go.
My trick for cutting shapes out of any tape whether it's duct tape, painters tape or electrical tape is to first put it on parchment paper and then cut out the desired shape.
We're thinking of putting glow sticks in the monster eggs for a treasure hunt in our yard this coming Halloween eve.