I saw this kit online recently:
Exclusive Dog Birthday Party Kit - GW Little
You can put the same thing together cheaper... buy a few stuffed toys and bags of treats for the "guests" as goody bags... buy some hats and decorations at a party store or the dollar store.
Make a little cake (dog friendly) they also sell one on that site.
Just have fun!
For my dogs birthday I usually just take them somewhere special, usually the dog park because they all have Spring/Summer birthdays... if it is raining/bad weather we might just go to petsmart or something and I let them pick a toy. Then they usually get something special for dinner, fish/sweet potato and peas is usually a big hit!