Kind of depends on the look you want
I had a nasty beige vinyl headliner and wall covering when the boat was purchased. Couldn't stand it, and it was very dark inside. I am now about 1/2 done with a new interior. Using a split bamboo for the walls. It's the stuff that is made and sold at Home Depot/Lowes for window shades. Inexpensive, fairly easy to put up and looks good with a couple coats of varnish. For the headliner I am using a paneling that looks like plank, painted white. This brightens up the interior of the boat a lot. Boat is much brighter inside, and looks a thousand times better than it did before. And I didn't break the bank to do it. Some of the carpet would probably be easier to install, if thats what you prefer. But look at the prices at places like Home Depot, as opposed to the "marine" places. Most of it is the exact same stuff, at less than half the price.