Wheels
Exercise wheels are a popular choice of accessory for many rodent owners, as they allow your pets to get a good workout even in the confines of their cage. However, although they are fantastic if you have a mouse, rat, hamster, or gerbil, they have been shown to be completely unsuitable for guinea pigs, for a number of reasons.
Exercise wheels are great for other rodents, but can cause serious injury to guinea pigs.
Back Pain and Spinal Injury
Unlike other small animals, a guinea pig's body is not designed to be bent backwards. Doing so can cause pain or even permanent damage. A guinea pig wheel forces your cavy's back into this unnatural, concave position, making it very uncomfortable to use even for a short period of time. Because of this, many guinea pig owners who do buy wheels find that their pets refuse to use them anyway.
Broken Bones
Many exercise wheels are made from wire mesh, meaning that your guinea pig is expected to run on thin wire bars with big gaps in between. As you can imagine, this is very difficult to do, and it is common for their feet to slip in between the bars while scurrying along at full speed. When your cavy's feet get trapped like this, it can easily lead to broken bones and other injuries.
Disorientation
Cavies have eyes on the sides of their heads. This gives them excellent all-round vision, but poor forward vision, particularly at close distances. When running in a guinea pig wheel, this makes it very difficult for them to see the wheel directly in front of them. As a result, even if your guinea pig can use its wheel without suffering pain, you will probably find that he avoids it anyway simply because he doesn't enjoy it.
Alternatives
So if you can't give your guinea pig a wheel, what can you give him? Some people buy a plastic exercise ball instead, but these cause exactly the same problems and so should be avoided as well. A much better option is simply to let your cavies run around freely on a flat surface. To help keep them contained, you could use a guinea pig playpen or run, both of which are perfectly safe.
Unfortunately, many pet stores still sell and recommend guinea pig running wheels, usually because they are unaware of the dangers they pose. I've experienced this myself, and usually find that they are grateful when I politely explain why they are unsafe. With a bit of luck they may even stop recommending them to other guinea pig owners, potentially saving other animals from the discomfort they can cause.