This is such a tricky question as needs change depending on season and age of ducks. Ducklings were hard. Constantly adjusting to their growth rates, mess rate, appetite ETC.
Here are a few things I have learned: My ducks free range during the day, year round. They have a pond in one corner of the yard and their night pen in another corner of the yard. Their pen door is left open during the day so they can come and go as they please, but locked up tight at night. A person with a different set up may have different needs.
For my adult ducks: Build the pen on high ground- reguardless of materials/ flooring used. Drainage & cleaning are easier.
Do not put a pond/ pool in their night-time enclosure. The biggest messes are water messes & cleaning it out is a nightmare...
Drainage under drinking water bucket/ feed dishes. Lots of people do not put food or water in the duck night pen. I do. I just cant have my babies going thirsty. In one corner of their pen, I dug out a hole, filled with rock & gravel covered with a plate of expanded steel, screwed to a 2x4 frame. (it is what I had but plastic would probably be better) this REALLY cuts down on the water mess as well as making clean up of the food area easier.
RAISE their food dish, but keep it secure from tipping. I made my own set up on 2x4's & screwed through the dishes into the board frame. It works for me. Less mess.
Use a bucket, 1-2 gallon, for water, Not a shallow dish, Not 5 gallon. smaller spills, gets poopy, gets swam in, while with larger people have reported drowning, but again, this is for ADULT ducks.
A place to hide, but not too deep.--- My ducks love hanging out in the dog houses in their night-time pen. They lay in there most of the time. One is no problem, the other one I need a rake to reach to the back of, on my hands & knees, in the poopy straw. Not much fun.
They love to nestle in the straw. -Use it somewhere. They search it for bugs & nestle. adorable.
Roof! they need a roof! In summer they need the shade, in winter the warmth and protection in all seasons. I alternated aluminum and plastic panels for the roof so it still lets light in but there is also shade. It turned out to work well for all seasons. !
Ventilation! As tempted as you may be to seal their pen up tight in the winter, they NEED the ventilation.
That is all I can think of at this moment.
Hope something here helps.