Animals in silly hats are a perennial favourite. Adults love them, kids adore them. So when my daughter required a birthday invitation, I gladly obliged her a goofy drawing of an owl in a birthday hat. It made for a fun invite, and even works as amusing postcard art.
Postcard style invitations are also a favourite of mine, because the size is right, they are simple to design, no folding, and they can double as tiny art pieces when the party is over.
This is a digital drawing, made in Procreate on my iPad Pro. I intentionally stepped away from including black in the colour palette to keep the illustration light and playful. Give that owl some cake!