Photo Streams on iOS are kind of a confusing and horrible way to manage photos. However, blogger Ben Walker points out that you can use shared photo streams as a massive storage system without using your iCloud storage space if you use the shared photo stream option.
It turns out that the shared photo streams do not count against your iCloud storage space. So, you can set up a shared photo stream, manually add pictures to it, and then you don't have to worry about storage space. The sharing limits here are huge. You can store 5000 pictures on your shared photo streams, but you can make 100 different streams. That means you get free storage for about 50,000 photos.
The downside is that this isn't automatic. You have to add photos to shared photo streams manually, but that's not a bad price to pay for a lot of free storage. Photo streams aren't exactly intuitive, but if you're on holidays and need to eek out a bit more space on your device, starting a shared photo stream might be the trick to do it.