I would go with a platform they could hop up to or walk a ramp up to. Dogs, like so many animals, like to survey the area. I have seen people hang a section of rope, the old kind, not synthetic, and let them jump at it, hang from it, bark at it. Balls, lots of balls even punctured are okay, those kind are easier to pick up. The inflated rolling ones of different sizes and weights, so they roll or bounce at different speeds. A floppy old rug they can drag around and tease each other with should also go over well.