Invites were made to fit into a regular #10 envelope.
Each invite was sent with a "Design Your Own Superhero Mask" activity page. You can download the printable version HERE.
But however was I to seal the envelopes to allude to the TOP SECRET within? How about some custom super easy stickers? I designed a return address sticker and ZAP, BAM, WHAM stickers to be printed on address labels (with Avery 5160 template). Download your own blank TOP SECRET label sheet HERE and ZAP, BAM, WHAM HERE.
I also made the birthday boy a t-shirt with iron-on transfer paper. I learned that it's best to print on a medium quality setting when using the "for dark fabric" transfer paper. I went through 5 attempts before the print stopped bleeding into the other colors.
I made him a cape with an adult XL t-shirt and the mask is two layers of felt, my beloved Heat-N-Bond, and 1/4in white elastic. I made about 12 of these for each of his friends and they were a crazy hit. I'll post a pattern soon!
Happy *belated* Birthday my little superhero!