Enrichment
Guinea pigs are quite relaxed little animals and find life less stressful than rabbits and chinchillas. However they will still enjoy having their lives enriched!
They are social animals so a companion is a great idea. If not, please spend lots of time with your guinea pig.
They are very keen on their food so a wide variety of vegetables will keep them happy. Make a foraging tray which you can fill with pieces of cardboard or old hay (hay that the best bits may have already been eaten but is not dirty or mouldy) and hide their vegetables in here so they need to search for them.
They are inquisitive and like exploring so let them out of the cage every day to explore, making sure there are no electrical cables they can chew on. Provide them with a hide box for when they need to sleep.
Always offer them chew toys made out of safe woods so they always have something to nibble on.