I like this challenge. I agree that everyday life can get quite boring, especially in winter time.
Here are some of my ideas:
You can find them some (non-poisonous) branches to chew.
You can buy a kilo of carrots and distribute them all over the pasture.
You can introduce playthings while you are with them, like a bag, poolnoodles, tarp, carton, pylons, barrier tape, umbrella, a blanket, a ball... Anything that won't hurt them. You can let them have a look, let them bite it and paw it and praise & reward every kind of interaction. Panti really loves that
I take the playthings out when we are finished.
Do you muck out your paddock? Then you can make a "wheelbarrow race" with your horses (run around with the wheelbarrow and optional mucking equipment banging around loudly).