An outside run is essential for regular exercise and grazing. Inactive guinea pigs develop cage paralysis as they get older. Be sure to provide shelter from weather and from possible predators.
Guinea pigs can have a run around inside your house, providing you don't have other pets which may be a danger to them. You must make sure they can't be injured, escape or get trapped. Electrical wires will need to be moved out of their way as will any house plant which may be harmful to them. Offer your guinea pig lots of safe hiding places. If you place some shallow cat litter trays filled with wood shavings and hay in your guinea pig's favourite hiding place they will normally use it as a toilet.
Move anything off the floor which you feel may be a danger to them; as with all rodents they have a tendency to gnaw.