I always approach birthday party invitations with the belief that the invitation sets the expectation. I like it to be in the spirit of the party, which always means fun! In my opinion, it should also be tied to whatever theme is being used to celebrate. Sometimes this is simply a matter of color palette but other times it may include motifs such as zoo animals, or a dinosaur, or a ticket to a county fair. I love printed cards and papers to i goo "all out" with my invitations--they are always snail mailed and often come in more elaborate forms such as a mini book or an accordion card. I design all my invitations myself so I can customize them, but there are fabulous invites to be found on Etsy, many of which include "editable text" or customizations to tailor them for your party. In addition to the essential details of day, date, time and location, I like to include a note about what else to expect--lunch? swimming? a trip to the zoo? Anything parents might appreciate knowing before they arrive.