Some scratch grains scattered over the coop litter for them to hunt for. Another roost put in somewhere, or if there's room, a small (stable!) table or such for them to explore sittin' on.
And, make the run as appealing as possible (windblocked, and perhaps straw put on the ice or snow) so they will spend as much time as possible out there. If they are young and won't go out it is worth manually chucking 'em out, with some food out there as something to DO, several days in a row to give them the idea. You can even lock them out for a couple hours at a time if they are just popping right back in immediately. Obviously you would exercise reasonable judgement about not doing this in a windy run on a -20 F day or etc, but, often it will make them considerably more outdoorsy.
A lot of people say 'hang a cabbage in the coop for them to peck at', and you can try that but mine have never had the slightest interest in it. Ditto the business of giving them a mirror.
Good luck, have fun,
Pat