1. Try putting apples, lettuce, carrots, etc. on a bit of string and hang it from your guinea pigs cage.
2. The more simple the toy, the better.
3. Take a paper towel tube and cut it open (this is so your guinea pig won't get stuck in it). Place it in your guinea pig's cage so they can chew on it. Hide some food in it while you feed them, it's great fun.
4. Take one or two paper bags for your guinea pigs to munch on and hide in, they are a big hit, for an even better experience, try cutting a little hole or peek hole in the back or on the sides.
5. Put your guinea pig's cage somewhere where people go by often. Remember, in the wild, cavies live in family groups, so guinea pigs can easily get lonely.
6. Build a small, sturdy shelf for your guinea pig, above the level of the plastic bottom of its cage, if it has that style of cage. It will be able to watch people go by.
7. Take a blanket or towel and bring your guinea pig outside the cage to let it run around.
8. Bring your guinea pig outside! If it's about 60-70ºF [no higher than 80ºF], and your lawn has not been treated with any chemicals, you can take your guinea pig out onto the grass. Guinea pigs love to be outside. Just watch them carefully, and if they show signs of overheating, bring them inside quickly. Watch out for hawks! [NOTE: Guinea Pigs also have a risk of hypothermia, always be careful and use common sense!]
9. If you own a hairless guinea pig, most of them are ticklish, tents made out of "clean" old shirts (be careful to remove all buttons) can be great fun, they love jumping in and out of the tent! If you are going to leave your guinea pig unattended with a tent, just make sure it's suspended securely, and not too much of the shirt is touching the ground, you don't want your little friend to get all tangled up.

Source: Wiki-How