Give degus as many branches to their cage as you can, they'll climb on them. Another good toy is running wheel, which must be at least 25 cm in average (degus can't run in smaller wheel). And some tunnels are also good, rather from wood than from plastic.
For smaller degus I give also coconut shells (without the content, dried). With short wire I hang it to the cage and degu babies sit in it, swing and sometimes they sleep there too. Unfortunately it's toosmall for adult degus, otherwise they would play with it too.