Okay, this is a fun question. I have hosted many doggie birthdays and have catered birthdays and puppy showers. Just like human parties, decide on a theme, and then go from there. One thing different, be absolutely sure that none of the dogs you invite are food aggressive. Typically a responsible owner knows if their dog is and will not bring them -- if there are any questions ask them to go to the time out area and eat separately from the other guests. By the way, it is good to have a separate, quiet time out room that dogs and their owners can go to if it gets to be too much. It is fun to have doggie bags filled with treats for the guests to take home with them. An easy way to make a cake is to take firm, bakery type whole wheat bread and cut into cake like layers. Fill with ground cooked liver (or liverwurst). Frost with a mix of cream cheese and unsweetened whipped cream. Decorate as for a human cake avoiding anything chocolate. Okay, here are some themes:
Mexican fiesta: Humans get mexican food. Dogs get a pinata filled with treats. Human kids get to hit the pinata with a bat, dogs scramble for treats. Party hats are sombreros
Treasure hunt: Hide bits of peanut butter or liverwurst sandwiches outside in a fenced yard. Dogs get to hunt for them.
Host dresses as pirate with parrot and eye patch.
Surprise party: Send out invitations but admonish dogs and their people not to "spill the beans".
Western ranch theme: Each attending dog gets a bandana to wear (preferably with their name on it). Have a BBQ for the humans. Have a weinie bob (hot dogs in big water container) Cowboy hats for humans and dogs