I'm thinking about getting a Remington 870 and I was wondering what length barrel to get. My dad has a 28" 870 and I like it a good bit but I was thinking of something shorter. I like the look of the 18" synthetic with the mag extension but I don't know if the accuracy would be affected. All I'm really gonna use it for is skeet shooting and just random "feel like shooting something" days. Maybe the occasional duck hunt if I ever decide to go. Thanks in advance for the help.

18" might be a little short for that.
Not because of any loss of "accuracy" but because for those types of shooting, longer barrels "point" better.

For what you want, a barrel from 22" to 24" inches would be good, especially if it's set up for screw in chokes.