Technology can improve learning on days that teachers have a substitute teacher. The teacher and the sub can both make online preparations to ensure a sick day is still a day full of learning.
Lets first consider the classroom teacher. There are any number of ways that you can prepare for a sick day. Traditionally, a teacher will have a subfolder prepared with a map, the teacher's schedule, trustworthy students and some simple lesson plans. If you know in advance, you can also have some worksheets printed off for the students to work on or an on-topic DVD to play for the class. These are all great, since you don't have to explain anything before they're able to take over your class. Plus, you don't have to worry about trouble shooting technology. However there are a few severe limitations as well. It's not dynamic for one. Unless you throw something out and re-print it, the folder will never be updated. It's also limited by size and location. You cannot put a year's worth of material in there, for example. And if it's ever lost, you'll have to make a new one.
A great digital alternative would be to make a class website. Today, there plenty of free website services. The great advantage here, is that you can update the site on the fly. You just need to tape a sign on your desk to tell your sub to go to your website and follow the directions. Here is an example. You'll notice that there are still a few documents that you'll have to have as a hard copy simply because of privacy issues.
Another great option is to have an Edmodo account. Edmodo is a way to safely stay in contact with students online. It also allows you to put class content online so that only authorized people can access it. For your sub, make up a fake name (Sub Teacher, for example), and give them access to class materials for the day. You'll also easily be able to stay in contact with your sub.
Now consider the experience from the sub's point of view. Occasionally, the lesson plans are lost or missing, which puts you in the awkward position of maintaining order with nothing for the students to do. Wouldn't it be great to have a few on-topic games on hand to play? Take a look at this list of interactive games. The links here are really designed to work with interactive white boards, but the vast majority of them work fine with just a computer and a projector.
You might not have access to a computer or projector, though. As long as you can get online (if you have a smartphone, for example), there is still help available. Check out this PDF. Its a list of closure activities that you can use to keep the students busy and learning at the same time. This is especially useful for when the plans that you are left don't provide enough for the kids to do. I recommend downloading it to your device so you have it handy as a resource.
Substitute teaching can run into a lot of snags, but hopefully these tips will help ensure a smooth and productive day.