Cage size done right, plus bedding, hides and the layout that makes for a happy, low-smell home.
Most pet-store cages are far too small. Guinea pigs are active and social, and space is the single biggest thing you control for their happiness.
For a pair, aim for at least 10.5 square feet (about a 2x4 grid C&C cage). Bigger is always better, and a roomy cage actually smells less and needs cleaning less often. Avoid multi-level 'ferret' cages, pigs are ground dwellers and can hurt themselves on ramps.
Two good options: washable fleece liners (cozy, cheaper over time, and much better for long coats) or a soft paper bedding. Skip cedar and pine shavings, the oils irritate their lungs.
