One of the easiest ways to decorate and up-cycle for any event is to grab a bunch of old cardboard, a sharpie, some watercolors and go for it.
Or better yet, get your kids to do it! You could even collaborate. "Hey child of mine, you're not SUPPOSED to, but.........do you want to use giant permanent markers to draw something for occasion X today?
"Oooh yes please" - they'll cry if they're anything like my child who will jump at the chance to do something she's not supposed to.
Then, if they need to go play tetherball or jump on a trampoline or play a game baby doggie in which one or more of them gets captured and put into a hard labor camp and has to be actually tied up for the duration of the game, well that's fine - because then you get to do the water coloring!
Water colour works great directly applied to cardboard or you could splurge and get some Crayon d'ache. They are these REALLY NICE french crayons, with prices that reflect the cost of each one getting it's own seat on the plane...flying first class, BUT ITS WORTH IT! They are water soluble things and are yummy squishy and loaded with pigment
You can use them as face paints too...at least I was told that when I worked at an art supply store years ago...and no one has had to be hospitalized yet.