Animals arent like us - they wont be miserable if they eat the same thing every day, and the bulk of their diet should be their duck feed, everything else is variety for the sake of variety, treats.
If you wanna grow some stuff indoors over the winter, grasses like wheat and rye are easy, clover is easy, lettuce (romaine, red leaf, salad mixes) are easy. You dont have to do anything fancy with them, just give them a container of dirt, sun and water and they'll grown like weeds.
I'd stay away from any canned veggies, there's added salts and sugars that arent great. Go for plain frozen stuff and just let it thaw to soften up before feeding (or steam/microwave to get it extra soft) Sometimes mashing it up or putting it in a bowl with a little water makes it easier for them to eat - my two didnt like banana the first time I tried it because I just mashed it, put it in water the second time and they went nuts for it. Fruits and berries also make good treats (remove pits and seeds, steer clear of citrus). Something treatworthy is usually in-season and on sale somewhere. But if you cant find something that wont break the bank that's perfectly okay, they dont NEED it as long as they're getting decent feed.
