If a kid's birthday party is going to be activity-oriented, a party favor can double as an invitation. For instance, if you have an adolescent Picasso who's hosting a painting party, send invitees a paint brush with their names written along the handle. Then, write the party details on a small piece of card stock (preferably shaped like a paint palette), punch a hole in it and attach it to the brush with string or ribbon.
You can do something similar with inexpensive magnifying glasses for a spy or science-themed party, tiaras for a princess party and sunglasses for a beach party.
Complementary Party Themes: painting, spy, science, adventure, beach or swimming pool, princess.