I squiggle curly cue designs with hot glue on plastic things all the time. Then paint it with whatever paint you can find for plastics outdoors. Doesn't last forever but then nothing really does.

The Krylon fusion paint works great for plastics and stands up relatively well.

John's idea of creating designs with a hot glue gun is also great. You can highlight the designs with a coordinating color by just rubbing over the raised sections. I've used a gold leaf on some of my plastic pots.

Speaking of glue, you can always cover them with moss, rocks, seashells, you name it. There are lots of possibilities

Thanks everyone for the ideas and the name of a paint. I like the idea of a raised design so maybe will try that.