Simply cute birthday party decorations can be made in a pinch with just white printer paper and markers, and of course some washi tape to hang your decorations! Using a black permanent marker, draw simple shapes and numbers that fill the entire A4 or letter sized paper, and then add details with regular markers in fun colors. Cut out your shapes and hang with the washi tape of your choice. That's it!
Keep this one in mind for that occasion when, yikes, you just realized that someone who has a birthday is coming to your home and there's still time to decorate a bit, but not tons of time.
Oh, and of course it does help if you already have a nice crocheted triangular bunting hanging on your wall for a bit of contrast, so consider crocheting one yourself to have on hand! (My pattern is here.)