As a reward for the garage having gasoline, bulb and sandwiches, I bought a celebratory air freshener for the car too.
In the evening we had a few celebratory noggins.
Different styles of party hats can be worn straight on the head, cocked at an angle, or even layered for an outrageous celebratory chapeau.
If that doesn't fit with the rest of your theme, maybe a different celebratory image like confetti or streamers would be appropriate.
Having live music from the quartet can really add to the celebratory atmosphere as guests relax and enjoy themselves.