Any kind. Seriously, anything can be made into a guinea pig toy. You can buy toys or make your own.
As far as homemade goes, paper towel tubes stuffed with hay, socks with a tennis ball or hay inside, or you could even sew your own toys. As for buying toys, you can get wooden chew blocks, hanging toys that they can bump around with their noses, stuffed toys that they can lay on and drag around, treat balls that you can fill with hay or veggies, and much much more. There's really no end to the amount of things a guinea pig can have for a toy.
Some of my girls' favourites are their stuffed carrot (Queen drags it all over with her!), their dangly thing that I attach veggies to, their hanging ball that I fill with hay or veggies, and their little dollar store pillows that they lay on. Little jingly cat balls are also popular, as well as baby plastic car keys! Lola also loves her hammock, although that may or may not be considered a toy.
There are tons of threads on toys on this forum. Do a search, you'll find tons of ideas and opinions on many different toys.
Edit: I forgot to add, I have a mini windchime hanging in their cage and they love it. They love to bump it with their nose and run when it makes noise! It's so cute, and it makes a nice sound, too.