Do you have a C&C cage? If you do not then you do not want many things in your pigs cage at all, as being a petstore cage it is already small, and by adding toys/houses you clog up the already limited floor space.
It is recommended to have some sort of hidy house in your pigs cage. Pigloos are one type of house you can use although personally I dont like these, as they tend to make the pigs more skittish, and instead I opt for a towel tent (where I attach a towel over one end of their cage making it hang down the sides and the back.
I also currently have a grid that I have bent in half making a tunnel in their cage. I ripped paper bags down the side so that it was flat when I opened it up, then I layed this over the bent grid and attached the paper bag to the grid with cable ties. The girls like to rip the paper bag off (shredding it in the process) and they also have a nifty tunnel.
Cuddle cups are also a hit in my household. I need to make some more as I want one for each of them so that they do not fight over the cups. Instructions on how to make your own cuddle cup can be found in the accessories forum.
Lazy lounges are a hit here, basically this is just a bent grid, cable tied to the side of the cage with a piece of corflute on the base making it hard, and then a soft cover like a towel.
Simple toys are generally more of a hit than the more detailed toys. Things like a paper bag, split down the middle and stuffed with hay keep them entertained for ages.
Also see this article
https://www.guineapigcages.com/toys.htm and this article
https://www.guineapigcages.com/accessories.htm