I took a box and cut the bottom of it off and cut holes for windows and a door and my guinea pig loves to play in it. he likes climbing in and out of the windows.
I also took a sock and filled it with tissue paper (or you could use actual paper) and I tied it shut (don't sew it because they'll chew the thread) and he went crazy over that he was wheeking while he was sniffing it and he was popcorning.
Guinea pigs like different textures such as fleece and other materials. They love things like a box of hay to tunnel through, areas of strips of fleece that hang down like an invrted forest (type 'fleece forest' into the search engine), a variety of tunnels of diffent shapes, lengths and materials......
They have individual tastes though, so what one likes another might not, but it looks like the vast majority like fleece forests and tunnels/boxes/hides and cosies made from fleece.