A long time ago as a sculpture student, I made 100s of cushions using calico and paint. (It was a work about interior decoration and the practice of buying paintings to match the furniture. The horror!)
I am someone who cannot throw anything into the rubbish without considering whether it can be reused, recycled or reimagined first. Which has meant those cushions have been dragged around for a long time. I have made all sorts of silly things with them, and just when I think they are now all gone, another bag of them turns up.
Lately I have been using them to make bunting. They are perfect for this: the colour being painted with housepaint, doesn't fade, they are fairly rigid, and a little heavy so won't fly around too crazily, and very robust, unlikely to mould etc and can be scrubbed. Perfect for outside (and inside too of course).
I cut out a whole heap of triangles with pinking shears. Mine are about an 18cm base and about 25cm long. Keep all the scraps and sew together into patchwork to make up more triangles.
You don't need to be complicated. Just sew in straight lines. Cut any excess seam so that they aren't too bulky at the back.
Then using zigzag stitch (I just like the look of it better than straight, but up to you). Sew one plain triangle to the back of one patchwork triangle. Sew wrong sides together as you are not turning it out or anything. What you sew is what you see.
Then when you have about 16 triangles at least, sew into the fold of some binding using a straight stitch this time. Leave about a 5cm gap between each flag. Sew alternating pattern to front, then plain. This makes the bunting double sided so looks good from all directions.
You also want at least 15 - 20cm binding at each end to tie bunting to something.
In terms of upcycling, think of using old paint, old testpots etc. Use acrylic. Only one coat necessary. Paint fabric with newspaper behind, as usually it will soak through. Again, as you are just doing block colours, children can easily help. If you don't want to make it a patchwork (is more work, but uses up all possible fabric so less waste) you can always paint your own patterns. Stick some making tape down. Use splodges and splatters. An old toothbrush makes a great splatter brush. I'd recommend painting outside for this as it would definitely be messy.
Use old curtains, old cushion fabric, old sheets, curtain lining, calico etc. The paint should strengthen to a degree any slightly deteriorating fabric.