If you got young ones, they also might like hurdles made from rulers, about 4-5cm (2 in) hich. When they are older, they prefer to push them out of the way.
Or you can simply drop an old towel on the floor with opening where the piggies can crawl under neather, or peg it to the side of the cage.
Guinea pigs do love things to investigate that smell of food. Try to swish a litle dish with some fruit juice and put it in their cage!
If you can create a cheap run, they will enjoy regular play time with toys out of their cage - and so will you! But you will need something waterproof as flooring if you live in rented accommodation, in case acidents happen and you need to make sure that they can get at any cables or in any nooks and crannies, which they will love to use at toilet corners!
PS: Stay off leads as well - while they are OK for ferrets, they can do serious harm to the much more inflexible guinea pig spines!