There are a number of party games well suited to a 50th birthday party, including "charades," "fictionary," "liar liar" and "name that tune." Some of these are better suited to adult players, whereas others are suitable for a range of age groups so the whole family can play.
Charades is an old favorite with widely varying rules. Generally speaking, players will divide into two teams and decide upon a word or phrase to be guessed by their opponents. This word or phrase is given to one member of the other team designated as the "actor," and they then have to mime clues for the rest of their team to guess. Lip movements are forbidden, as are sounds.
"Fictionary" involves one player, the "picker," choosing an obscure word from the dictionary and spelling it out for the other players. Everyone writes out their own definition for the word and hands it to the picker, who shuffles them together and reads them back to the group. The winner is the person who guesses which definition is correct most consistently.
"Liar liar" is a simple game in which everyone tells an outrageous story, only one of which is false. Players have to guess which one.
"Name that tune" is another simple game, which can be personalized to the person whose birthday it is. Songs may be recorded from the year of their birth and played back in fragments for players to guess.