I don’t tend to buy toys, I find them to be a waste of money as mine just aren’t interested. What they do like though is
anything where food is involved so empty toilet roll tubes cut in half lengthways and stuffed with hay. Cardboard boxes, doors cut in as houses and stuffed with hay (Sometimes with veggies hidden in). Just big piles of hay with their pellets, forage or veggies hidden amongst. The only things which I buy are carrot cottages, haycubes and large cardboard tunnels as they do like them.
Some other ideas.
Enrichment Ideas for Guinea Pigs