you could use a long outdoors dog leash (a tie out leash) with a harness. I used to use a regular diaper harness with no diaper insert in it with a tie out leash when i brought my ducks to a park that had a pond (back when i lived in AL). I'm working on something a duck could wear that has no butt pouch, but the process is slow going.... sorry
But you could do some activities for them... Like get a couple of cheapy small cat liter pans and make "activity centers" for them (lol like what i do in my preschool classes, this is where i got the idea)....
In the different pans you can put in different things for them to play with.
In one, you could add sand, and mix in some dry treats like cat food for them to search for.
In another you could put in gravel and some seeds, (like beans, corn, grass, clover, etc) and water. Let them grow for a few days and the let the ducks have at it.
In another you can put in some soil (with no fertilizer) and add earthworms, greens, etc for them to search through.
You can also put crickets in a tall glass container for them to eat.
In a tall glass container you can also mix water and brine shrimp from a pet store... Mix the cube of shrimp into the water. the ducks will have tons of fun sifting them out.
They also like any toys that have bells, anything that crinkles, anything shiny (my ducks have a disco ball thing), anything that rattles, or mirrors. Buy a few different toys and change them out every other day or so to keep things fresh.
you can play a few games with them... I used to play a game called "pitter-patter get the peas!" where i just thaw some frozen peas, and roll them down my patio or my frontroom floor and the ducks go running after them. I throw some far away and then some close or in other directions forcing them to run around to get peas. I was also able to teach my ducks to jump up to get a pea held between my fingers, so that's something you can try with them.... I also taught them to bow their heads by holding a pea on the ground in front of them. When you hold the pea down, say a command like "wait" or "bow" or "nod". eventually they will get the hang of it. At least it gives you something to do with them!