Pets like their toys, and guinea pigs are no exception. However, they do get bored easily if they only have the same toys to play with over and over again, so it's nice to give them a lot of variety.

There are some toys made just for guinea pigs, such as Bounce Back toys (you place a treat inside the toy and the cavy knocks it down to get to the treat, then it bounces right back up again). At only $6 or so, it's an affordable purchase and most guinea pigs seem to like it.

Of course, you don't have to go out to your local pet stores in order to gather toys for your guinea pig. Many items that you have lying around your house would also be perfect. Let me give you some examples:
Paper - Guinea pigs love to chew on and play with any kind of paper, including the newspaper. If you do give them paper to play with, make sure it is as plain as possible since the cavies will probably be ingesting some of it.
Cardboard spools- You know those spools inside your rolls of toilet paper and paper towels that you normally just throw away? Don't throw them away any more. These are great toys for your guinea pigs.
Old Clothing - Socks, towels, even small shirts can be added to your pet's cage so that they can hide in them or use them as pillows.
Small balls - While guinea pigs usually love those cat balls with the bell in the center, they aren't the best toys for your pet. A tennis ball or ping pong ball will deliver the same fun but without the danger.
Some children's toys - Small stuffed animals, old-fashioned wooden blocks, and similar items can be passed down to your guinea pig for hours of amusement. Be sure to watch the guinea pig so that he or she doesn't start eating the inside part of the stuffed animal which can be dangerous.
PVC Tubing - If you have some old pieces of plumbing pipe around, you can put it into the cage. The guinea pig will usually hide under it or push it around the cage.

There are two things you should not allow your guinea pig to play with even though they are regularly sold in pet stores and may seem ideal for your pet: exercise wheels and balls.

While smaller versions of both of these are commonly enjoyed by hamsters, gerbils, and rats, the larger versions are not good for guinea pigs. Both can actually cause serious injuries to the cavy's feet, legs, even back. Plus, the exercise balls have a habit of popping open which could give the guinea pig enough time to escape.

If you want to safely allow your guinea pig to get exercise while staying in a confined area, you are much better off purchasing a small pet playpen which you can set up in any room of your house, then covering the floor with newspaper, and allowing your pet to run around or to play with their ordinary toys inside of the fenced in area. Its much safer for the guinea pig.