Some guinea pigs like to play with their hay racks, too, tugging on long strands of hay and pulling them out for fun. Some people like to mount a salt or mineral wheel in the cage. Although it's not necessary for a guinea pig, some pigs do like them, and they last practically forever. For terrain and variety, many people like to add rocks, bricks and tubes to their cage. These not only provide rough surfaces for wearing down claws, but also hiding places and things to climb on or around to help alleviate boredom.
Bricks are especially cheap and rocks are often times free.
The PVC plastic is hard, durable and easy to clean. Some guinea pigs even like bird toys, particularly the mirrors with bells around them. In addition, some owners will put in old towels or cloths for the guinea pig to hide or nest in.