7. Hiding place
Although guinea pigs have been domesticated for quite some time, they still feel a need to have a place to hide from potential threats, real or imagined. You have to remember that despite being in a relatively safe home, they still retain the primitive instincts that once saved them from large predators.
Pet stores carry a variety of custom huts that would be ideal for your guinea pig, but if you have the materials to make one yourself, you might be able to create something a bit more special.