You can stuff toilet paper rolls with shredded paper and pieces of millet spray. It would make a mess, but they would have fun.

Those ropes look somewhat flimsy though. I would be concerned about them coming undone. If you want sturdier ropes, you can get some rope perches. They're moldable, and stay in place, and would probably be better than just regular ropes.

It would also probably be best to have a at least a couple wood perches. Sometimes, birds can get their toenails caught in ropes, so you really wouldn't want them to be the main types of perches. The best kinds are the kinds that aren't completely straight nor even. They simulate tree branches and are very good on the bird's feet. Having a few different widths is also important, as their feet tend to cramp up when sitting on the same-sized perches all the time.

At the same time, you don't want to crowd their cage too much, as cockatiels like to stretch and flap their wings.
Other than that, all I can say is, as Carolyn said, make sure you have a reliable person to take care of them while you're gone, to make sure they have fresh food and clean water.
I giggled at Coco at the end, and how he tried to eat the camera. Lucy does the same exact thing. I usually have to take pictures while she's distracted, or else I'll end up with more pictures like this:

This one was actually taken intentionally, which is why it is pretty clear. Otherwise, they end up really blurry.