For my daughter's 5th birthday, she asked for a Beatle's yellow submarine party! We decorated the yard with Beatle's posters and under the sea type decorations, played pin the glasses on John Lennon, played musical chairs to Beatle's music.  Each child had to search for 3 plastic submarines of a specific color/design (which were hidden around the yard). When they found all 3, they received a blow up instrument .. then we played our instruments to Beatle's music and recorded our concert!   The goody bags (blue bags decorated with yellow submarines) contained John Lennon sunglasses and yellow submarine trading cards.  I was told by many of the parents that it was the best 5 year old party they had ever attended!