Degus are very intelligent creatures, and they seem to enjoy having their intelligence challenged. Give them a puzzle with a treat at the end of it and they'll do their best to get that treat. All these tests are designed to stimulate your degu into learning new tricks to get results, and they'll do fine without them but it's nice to give them something new to do every so often.
The hidden treat in the toilet roll
Put timothy hay in one side of the toilet roll filling it halfway and put some treats in the other end and fill it up the rest of the way with timothy hay, so the treat is right in the middle...now put it in the cage so they can see the toilet roll and smell the treats but not see the treat...they'll work out where the scent is coming from and dig their way through the timothy hay to get at it.
More intelligence tests to come soon...feel free to add your own!