My nephew had a birthday and of course I was super late and couldn't think of anything fun or cool and finally decided that we would mail him a complete birthday wrapped up in a box. I gathered a small box, a couple of gifts that would fit, a tiny homemade birthday banner, loads of confetti, a few not-yet-blown balloons, and even a little "cake" ( I thought about a candle but figured it might melt). Then I tied it all up with twine, because boxes look so great with twine.
We also added a quick little card to the front. Now I can't stop thinking about birthdays in boxes and all of the different combinations/themes/colors. Just think. A birthday in a box for a little girl with sparkly pink things... one for a friend with lip gloss, earrings and lotions... or a new baby "birth"day in a box with a newspaper clipping from that day, a binkie, maybe a fun onesie, some hand sanitizer...etc. Oh the possibilities.
We gave him a small airplane/glider kit and a pack of gum (since kids always love gum).