Have you tried treats? My ducks will come anywhere with me for peas or mealworms, and it might be an easy way to keep them in your reach on the creek. Might be much more fun than leashes. I try to move my ducks around a bit between their run, foraging, and their grassy tractor. I know that the younger they are, the more easily bored and energetic they are. The older they get, the more content they are to not move around so much. I would hesitate to take them for walks - depends so much on the surface their feet are on - rocks and concrete can be too hard and cause bumbles. Keeping them on grass and soft surfaces as much as possible is great. When I put my ducks in their portable "tractor" on the lawn often sprinkle mealworms all around on the grass and they spend a lot of time hunting them down. Also, since you only have two, you might consider getting a few more next spring so they have company. The rough part about two is that if lose one, the other one will be devastated, and ducks have so many potential health issues, it's very possible.