Fleece bedding comes in many fun prints and can instantly give your cage a new look. Try mixing and matching different patterns and making your cage themed. For more info on buying fleece and more, check out this handy guide from Cali Cavy Collective.
Guinea pigs enjoy having a place to hide in so you may want to invest in things such as pigloos, tunnels, cozy bags/sacks, plush houses, or even a small hammock! Spiffy Pigs on Etsy has a great selection of beds, tunnels, and cozy sacks and you can visit their store here. We also recommend Coziehedgiebags, another store on Etsy that also various cage accessories such as cage liners and play tunnels. The store sells items that are actually meant for hedgehogs, but some guinea pigs can also use them too.
If you are crafty and enjoy sewing, you could also make your own cage accessories. We found this fun tutorial on how to make a "mushroom den" pictured below. The instructions can be found here on Craft Wookie.
Some other ideas for decorating your cavy's cage include adding small toys, wood chews, pillows and cushions, a mirror, fleece curtains, and chew toys. Happy Cavy has a great post featuring cheap and almost-free toys for your guinea pig right here.