Access to everything they require at all times (e.g. space, food, water, safe hiding places, companion guinea pig(s), toys).
Safe toys to play with/chew, and regular opportunities to play with other friendly guinea pigs or people. They're inquisitive; if bored, without enough to do, they may suffer. Guinea pigs are very social and need to interact with other friendly guinea pigs. Many enjoy interacting with people.
Constant access to safe hiding places, pipes and shelters, to be alone/hide/escape if they feel afraid. There must be enough places for all to hide simultaneously. Guinea pigs must be able to avoid things that scare them. As they're a prey species, they must be able to hide in a secure place, away from the sight and smell of predators (e.g. foxes/cats/dogs/ferrets/birds of prey).
Opportunities to exercise daily to stay fit and healthy. Guinea pigs are active animals, during the day and night and need frequent opportunities to exercise.
Suitable materials that allow tunnelling behaviour, such as pipes and deep areas of hay.
You to be observant. If your guinea pigs' behaviour changes or shows signs of stress/fear, seek advice from a vet or clinical animal behaviourist - they could be distressed, bored, ill or injured. Guinea pig's behaviour depends on age/personality/past experiences. Guinea pigs that are frightened/in pain may change their behaviour/develop unwanted habits e.g. aggression/hiding. Signs a guinea pig may be suffering from stress/fear can include hiding most of the time, chewing cage bars, over-grooming, altered feeding or toileting habits, over-drinking or playing with the water bottle, sitting hunched, reluctance to move, and repeated circling of their enclosure.
Kindness! Be quiet and gentle around them. Never shout at or punish guinea pigs, they are very unlikely to understand and can become more nervous/scared. If your guinea pig¿s behaviour becomes an ongoing problem, seek expert advice from your vet or a clinical animal behaviourist.