I’d probably get some driftwood since they require it for their diet, and maybe get some long pieces of wood that have a hollow center that is aquarium safe. Otherwise, try to make some type of blacked-out area so they can escape from any light source if needed.
Definitely wood and plenty of it if you are able to. Plecos love to hang around wood as it's their natural habitat and where they most feel at home. In terms of what wood you choose I would say go for something that you would love to see in your tank. They will be happy with any kind of wood and there are plenty to choose from as well.